The film takes place in two different times: the present and 11 years earlier. The two plot lines are told in parallel through flashbacks. In 2002, software engineer Alan Russell moves into a new house with his wife Marie, 10-year-old son Tim, and 12-year-old daughter Kaylie. Alan purchases an antique mirror to decorate his office. Unbeknownst to them, the mirror supernaturally induces hallucinations. Marie is haunted by visions of her own body decaying, while Alan is seduced by a mysterious and ghostly woman named Marisol Chavez who has mirrors in place of eyes. In 2013, Tim is discharged from a psychiatric hospital, having been convinced that there were no supernatural events involved in his parents' deaths. Kaylie has spent most of her young adulthood researching the history and perceived powers of the mirror. Using her position as an employee of an auction house, she obtains access to the mirror and has it transported to the family home, where she places it in a room filled with surveillance cameras and a "kill switch" — an anchor attached to the ceiling on a pivot, positioned to swing down and smash the mirror. Kaylie intends to destroy the mirror, but first wants to document its power, proving both Alan's and Tim's innocence. Over time back in 2002, the parents become psychotic: Alan isolates himself in his office and Marie becomes withdrawn and paranoid. All of the plants in the house die, and their family dog Mason disappears after being shut in the office with the mirror. After Kaylie sees Alan with Marisol, she tells her mother and the parents fight. One night, Marie, distraught over her husband's supposed affair, tries to harm the mirror, finally becoming possessed by it, and attempts to kill her children, but Alan locks her away. When the family runs out of food, the children realize that their father is now permanently under the influence of the mirror too, so Kaylie goes to seek help from their mother and finds her chained to the wall, acting like an animal. Kaylie and Tim try going to their neighbors for help, but the neighbors disbelieve their stories. When Kaylie attempts to use the phone, she discovers that all of her phone calls are answered by the same man. The fully possessed Alan eventually loads up a gun previously purchased by him and unchains Marie, with both parents attacking the children. Marie briefly comes to her senses, only to be shot dead by Alan. The children try to destroy the mirror but it tricks them, making them believe they are hitting the mirror when they are actually hitting the wall. Alan also experiences a moment of lucidity and kills himself by forcing Tim to pull the trigger of the gun and shoot him, causing a small crack in the corner of the mirror in the process. Before dying, he begs the children to run, but Marisol and other victims of the mirror appear as ghosts. The police arrive and take Tim into custody. Before the siblings are separated, they promise to never forget what happened with the mirror, one day reunite as adults and destroy it. As Tim is taken away and put behind bars, he sees the ghosts of his parents watching him from the house. Back in 2013, Tim attempts to convince Kaylie that she is wrong and the siblings argue. When they notice the houseplants begin to wilt, they review the camera footage and see themselves performing actions they do not remember doing. Tim finally accepts the mirror's supernatural power and attempts to escape the house with Kaylie, only for the pair to be drawn back by the mirror's influence. Seeing a hallucination of her mother, Kaylie stabs it in the neck, only to realize that she has stabbed Michael Dumont, her fiancé and one of her fail-safes, who has come to check on her. They try to call the police, but are only able to reach the same voice who spoke to them on the phone as children. At this point, they see their doppelgangers inside the house standing in front of the mirror. Realizing that the 9-1-1 call is not going through, they go back inside the house. Kaylie and Tim begin hallucinating by seeing younger versions of themselves, the past and the present merging into each other. They are separated and each of them relives the nightmare from their youth. Tim awakens alone in the room with the mirror, while simultaneously a younger Kaylie hallucinates her mother beckoning her from the mirror. Tim releases the anchor, unable to see that Kaylie is standing before the mirror, and inadvertently kills her instead of destroying it. The police arrive and arrest Tim, who is hysterical, just as they did when he was younger. As both a boy and an adult, Tim claims the mirror is responsible. As he is taken away, Tim's adult incarnation sees Kaylie's ghost standing in the house with his parents.

In 1984, young guitarist Euronymous forms black metal band Mayhem, the first of the genre in their country of Norway, with Necrobutcher on bass and Manheim on drums. Manheim leaves and is soon replaced by new drummer Hellhammer, and they recruit a vocalist from Sweden called Dead. Dead exhibits self-destructive behavior; during their live shows, he cuts himself, bleeds on the audience, and throws pig heads at the " posers ". At a show filmed by their friend Metalion, the band meets a timid fan named Kristian, whom Euronymous initially undermines and patronizes. Euronymous also meets Ann-Marit, a girl from the metal community, and is instantly attracted to her. While home alone, Dead slits his wrists and throat with a knife, then uses Euronymous' shotgun to shoot himself in the forehead, leaving behind a suicide note. Euronymous returns to find Dead's body; instead of calling the police, he repositions and takes photos of the corpse. After Dead's body is taken to the morgue, Euronymous makes necklaces out of pieces of Dead's skull and gives them to the other members. Horrified and disgusted at Euronymous' flippant response to their friend's death, Necrobutcher leaves the band. Soon after, Euronymous starts his own black metal record label and opens a record shop called Helvete ("Hell"), which becomes a social hub for black metal fans, including Kristian, who is now calling himself Varg Vikernes. They become known as the "Black Circle". After being mocked by Euronymous, Varg burns down a local church. When challenged by Varg regarding his status as the leader of the Black Circle, Euronymous burns down a church with Faust and Varg accompanying. Not long after, Ann-Marit and Euronymous enter a relationship. Euronymous recruits Varg as bassist, along with a guitarist called Blackthorn and Hungarian vocalist Attila Csihar, to record Mayhem's first album, De Mysteriis Dom Sathanas. A power struggle between Varg and Euronymous soon arises. After a wave of church burnings and the murder of a gay man by Faust, police begin to link members of the local black metal scene to the crimes, sending Euronymous into paranoia. Helvete is shut down, and Varg is arrested as prime suspect after an interview with a Bergen newspaper in which he boasts about his responsibility for the crimes, though he is soon released due to a lack of evidence. Afterwards, Varg tells Euronymous that he is leaving Mayhem with plans to start his own record label. In the ensuing conversation, Euronymous admits that the skull piece necklaces were fake and that his violent, anti-authority mindset is merely a persona he adopted for the sake of promoting the band, which angers Varg. While packing up the shop, Euronymous angrily rants to a peer about Varg, blaming Varg for the deterioration of the black metal community and expressing a desire to kill him in retaliation. After taking advice from Ann-Marit, Euronymous calms down, sends Varg a contract releasing Burzum 's music rights to him, and decides to continue with Mayhem. When Varg hears of Euronymous' death threats, he travels to Oslo to confront him. He convinces Euronymous to let him enter the apartment by stating he wants to sign the contract. However, once inside, Varg stabs him after a brief conversation. Varg chases Euronymous to the stairwell outside, where he stabs him to death, ignoring him as he pleads for his life. News of Euronymous' murder spreads throughout Norway and Varg is soon arrested. He is convicted of both the murder of Euronymous and the burning of several churches and sentenced to a maximum of 21 years in prison. In a voice-over, Euronymous tells the audience not to mourn his death, as he lived a successful life and accomplished many things.

Medical student Nelson Wright convinces classmates Joe Hurley, David Labraccio, Rachel Manus, and Randy Steckle to help him discover what lies beyond death. Nelson flatlines for one minute before his classmates resuscitate him. While "dead", he sees a boy he bullied as a child, Billy Mahoney. He merely tells his friends that he cannot describe what he saw, but something is there. The others follow Nelson's feat. Joe flatlines next and experiences an erotic sequence linked to his promiscuous lifestyle. After arguing with Rachel and out-bidding her of the length of time that they are willing to remain “dead”, David is third to flatline on Halloween and sees a girl, Winnie Hicks, whom he bullied in grade school. The three men later start to experience hallucinations related to their visions. Nelson gets physically assaulted by Billy Mahoney twice. Joe, engaged to be married, is haunted by the women that he secretly videotaped during his sexual dalliances, who taunt him with the same false promises he used on them. On a train, David is confronted by the 8-year-old Winnie, who taunts him the way he taunted her. Rachel decides to flatline next. David rushes in, intending to stop the others from giving her their same fate, but arrives too late. Rachel nearly dies when the power goes out and the men cannot shock her with defibrillator paddles. She survives, but she too is haunted by the memory of her father dying by suicide when she was young. The three men reveal their harrowing experiences to one another, and David decides to put a stop to his visions. Meanwhile, Joe's fiancée, Anne, comes to his apartment and, having discovered his videos, ends their relationship. Joe's visions cease after Anne leaves him. David goes to visit a now adult Winnie and apologizes. She accepts his apology and thanks him, who feels a weight lifted off his shoulders. David then finds Nelson, who accompanied him to visit Winnie, beating himself with an axe. In Nelson's mind, Billy is again attempting to beat him to death. David stops him, and they return to town, where Rachel confronts Nelson about withholding the supernatural nature of the experiments from the rest of them, then storms off. David later instructs Joe and Randy to help Nelson find Billy. Having an idea of what Rachel has experienced, David offers to let her stay with him and they fall asleep together. Meanwhile, Nelson takes Randy and Joe to a graveyard. He killed Billy as a kid by throwing rocks until he fell out of a tree. They try to convince Nelson that what he did was accidental, but he does not listen. They are eventually stranded when Nelson storms off in Joe's Mustang. David leaves Rachel to rescue Joe and Randy at the cemetery. While alone, she goes to the bathroom and finds her father. He apologizes to Rachel, whose guilt over his death is lifted after he reveals he was addicted to morphine and his suicide was related to post-traumatic stress disorder resulting from his tour in Vietnam. Nelson calls David's house, and when Rachel answers he tells her he needs to flatline again to make amends. He apologizes for involving her and their friends in his reckless plan. The three men realize what he intends and race to stop Nelson, who has been dead for nine minutes when they arrive. Together with Rachel, the four friends work to save him. In the afterlife, the boy Nelson is in the tree being stoned by Billy from the ground and dies from the subsequent fall. When almost all his friends are about to give up on reviving Nelson, Billy forgives him. David gives Nelson one last shock, which brings him back.

In 19th century Europe, nobleman Kurt Menliff returns to his family's seaside castle after many years of absence in order to supposedly congratulate his younger brother Christian for marrying his former lover Nevenka. However, those at the castle know that Kurt is a sadist and are unhappy with his presence, especially his father Count Menliff and housemaid Giorgia, the latter of whom blames Kurt for the suicide of her daughter Tania, with whom Kurt had an affair before abruptly abandoning her. Nevertheless, they decide to let Kurt stay at the castle overnight. The next day, Kurt finds Nevenka sitting by herself on the beach. After seducing her, they kiss, but Nevenka suddenly backs away in a panic and strikes Kurt with a riding crop. In response, Kurt takes the crop and whips her on the back several times, to which Nevenka apparently reacts with sexual excitement as she and Kurt make love immediately afterwards. In the evening, after Nevenka fails to return to the castle, Christian sets out to look for her alongside butler Losat and Kurt's cousin Katia, who is secretly in love with Christian. Meanwhile, an unseen figure stabs Kurt in the throat with the dagger that Tania used to kill herself. Losat finds Nevenka lying unconscious on the beach, and he and the others take her back to the castle and put her to bed before discovering Kurt's body. After Kurt's funeral, the relationships between Nevenka, Christian and Count Meniff begin to fall apart, as Christian suspects his father of having killed Kurt, and Nevenka overhears Christian declaring his love for Katia. Nevenka also begins seeing Kurt in numerous ghostly visions, including one in which he whips her several times in her bedroom, leaving welts all over her body. Eventually, Count Menliff is found dead in his bedroom, having been killed in the exact same way as Kurt. As the castle occupants start to suspect each other of killing the Count, Nevenka suddenly disappears from the castle and is found in the family crypt in an apparent state of ecstasy, claiming that Kurt carried her to the crypt and whipped her. As Christian, Katia and Losat also start having supposed visions of Kurt, Christian comes to believe Nevenka's claims that Kurt's ghost is the one behind the Count's murder, so he and Losat decide to unearth and cremate Kurt's body in an effort to bring an end to the hauntings. While burning the body, Christian and Losat hear a sinister laugh and spot a figure in a cloak spying on them. Christian chases after the figure, who is revealed to be a crazed Nevenka, who attempts to stab Christian with the dagger before fleeing back into the crypt. There, she has another vision of Kurt, and attempts to stab him, but inadvertedly stabs herself, much to Christian's dismay. Before dying, she reveals to Christian that she has "killed again, this time for good." Losat and Christian determine that Nevenka had gone insane or was perhaps possessed after murdering Kurt, leading her to kill the Count and stage the various "hauntings". Meanwhile, as Kurt's body continues to burn, a riding crop placed in his coffin appears to move by itself.

During a July 2020 COVID-19 lockdown in London, friends Haley, Jemma, Radina, Emma, Caroline, and Teddy join the weekly Zoom call they have been using to stay in touch. Haley has arranged for them to partake in a virtual séance led by the medium Seylan, who emphasizes that they should not disrespect the spirits, though only Haley and Caroline takes it seriously. Teddy is forced to leave the call when his girlfriend Jinny disconnects him. Jemma claims to feel intense tension around her neck and begins to cry, explaining that she feels the presence of a school friend who killed himself by hanging. Seylan's internet cuts out and disconnects her from the call, prompting a laughing Jemma to admit that she made the story up because she was bored, which angers Haley. While they are arguing, Haley's chair is suddenly pulled across the room, to the shock of the group. A series of terrifying phenomena commence: the legs of a hanging corpse appear in Caroline's attic when she goes to investigate a noise; when Haley points her instant camera down the hallway to take a photo of her living room, it shows a similar figure hanging from the ceiling; and Emma's wine glass shatters untouched. As the girls panic, Haley gets back in touch with Seylan and explains the situation. Seylan explains that Jemma's prank, fabricating a deceased person, may have led a demon to take on the guise of the deceased person and the identity like a mask, allowing the demon to pass into the living's world. She instructs them to close the séance, but the demon interrupts her and she is disconnected again. The girls attempt to close the circle following her instructions. Relieved that the ordeal seems to be over, the group members begin preparing to leaving the call, when Caroline suddenly appears to be flung into the camera. Emma, forgetting to switch off a filter, turns her camera towards her living room, where the filter places a mask on the face of an invisible figure which then turns to look at her. She scatters flour on the floor, in which footprints approaching her appear, and she is attacked before escaping into her bedroom. Alan's body drops down behind Radina, prompting her to flee, but the demon kills her. Caroline's camera turns back on to show the demon smashing her head into her keyboard while she pleads for help before cutting out. Haley is pulled backward through the door of her room. Jemma, who lives around the corner from Haley, rushes over. Teddy returns to the call to see that only Emma remains. Unaware of everything that has occurred, Teddy believes Emma is playing a prank on him, but he is attacked by the demon after it takes the form of a horrifying humanoid figure. He is chased into his garden, where Jinny is lifted into the air and has her neck snapped. He runs away and hides, using a lighter to see where he is going, but the demon distracts him with an eerie music box his brother used to scare him with as a child. The demon knocks him down and he drops the lighter, causing a fire that burns him to death. Emma's bedroom door opens, and she throws a blanket which drapes itself over an invisible human shape. She opens her window to escape, and falls to her death. Jemma breaks into Haley's home, and the demon smashes a bottle against her head and starts to destroy the kitchen, but Jemma recovers and finds Haley hiding under her desk. They attempt to escape the house using the flash from Haley's camera to light the way. The demon appears in the final flash of light, and rushes at them as the Zoom call timer expires.

In 1974 Oregon, a young girl with a Polaroid camera follows a mysterious voice and encounters an erratic man in pale makeup. Twenty years later, FBI agent Lee Harker is assigned by her supervisor, William Carter, to a case involving a series of murder–suicides in Oregon. Each case consists of a father killing his family, then himself, leaving behind a letter with Satanic glyphs signed "Longlegs", whose handwriting belongs to none of the family members. Lee exhibits possible clairvoyance and manages to decode Longlegs' letters. Further investigation reveals that each family had a 9-year-old daughter born on the 14th day of the month. The murders all occurred within six days before or after said birthday, and their respective dates form an occult triangle symbol on a calendar, with one date missing. While at home talking on the phone to her mother, Ruth, Lee receives a coded birthday card from Longlegs, warning her that revealing its source will lead to her mother's murder. Following a clue, Lee and William discover a doll containing a high-energy metal orb inside its head. After visiting a mental hospital to question Carrie Anne Camera, the sole survivor of Longlegs' murders who was visited previously by someone using Lee's name, William suspects Lee has a connection to Longlegs. Discovering that Ruth had filed a police report of an intruder approaching Lee the day before her 9th birthday, William instructs Lee to visit her. Ruth directs Lee to her childhood belongings, where she finds a Polaroid that reveals Longlegs to be the man who visited a young Lee on her birthday in the opening sequence. Lee submits the photo, leading to Longlegs' arrest. Realizing the missing calendar date is that day, Lee fears an unknown accomplice of Longlegs will carry out the murder. In an interrogation room, Longlegs claims to serve " the man downstairs " and hints at Ruth's involvement in the murders before taking his own life by repeatedly bashing his face and forehead onto the metal table. Shortly after leaving the interrogation room, Lee is informed by William that Carrie Anne has committed suicide. Agent Browning drives Lee to Ruth's home. Browning waits in the car outside while Lee searches the house, but Ruth approaches the car and fatally shoots Browning before destroying a doll resembling a young Lee, causing Lee to lose consciousness. In a vision, Lee discovers that she had been threatened by Longlegs as a child, who only spared her after Ruth pleaded for her life. In return for sparing Lee, Longlegs demanded Ruth's servitude; Ruth agreed, allowing Longlegs to live in the Harker basement and create Satanic dolls that Ruth, posing as a nun, delivered to households, causing each family's patriarch to commit familicide. Lee's doll blocked her memories of Longlegs while influencing her with his magic. Lee awakens in the basement and answers the phone, where a demonic voice warns her about William's daughter Ruby's 9th birthday party, scheduled for that day. Lee rushes to save the Carters, whose deaths would complete Longlegs' triangle. She finds Ruth has already delivered the doll and possessed the family. After William murders his wife, Anna, Lee fatally shoots him to protect Ruby. Ruth lunges at Ruby with a dagger, forcing Lee to kill her. Lee tries to destroy the doll, but her revolver misfires. She yells for an unresponsive Ruby to leave, who continues only to stare at the doll.

To escape a stalker, Sawyer Valentini moves away from her Boston home. Still paranoid and traumatized, she talks with a counselor at Highland Creek Behavioral Center, who tricks her into signing a consent form for voluntary 24-hour admission to a locked psychiatric hospital. Sawyer calls the police, who can do nothing due to the signed form. During the night, stress causes Sawyer to lash out at a patient and a staff member. Consequently, the staff psychiatrist retains her for seven more days. Another patient, Nate Hoffman, reveals to Sawyer that Highland Creek is running a scheme to exploit health insurance claims. They trick people into voluntarily committing themselves as long as the patients' insurance companies continue to pay; when insurance claims run out, the patient is "cured" and released. One day, Sawyer sees David Strine, her stalker, working as an orderly under the pseudonym George Shaw. Borrowing Nate's secret cellphone, Sawyer calls her mother Angela. David gives Sawyer a large dose of methylphenidate, causing her to appear insane. That evening, when Angela arrives to attempt to get Sawyer out, David approaches her posing as a hotel employee, and kills her. David tortures Nate then kills him with an overdose of fentanyl. Sawyer finds Nate's phone under her pillow, with images of Nate badly beaten. She alerts the staff, who dismiss and put her in solitary confinement. David visits Sawyer and says he has a secluded mountain cabin he wants to take Sawyer to. Sawyer mocks him for his inexperience with women. David later returns and reveals he faked that Sawyer's insurance ran out, changing her status to released. In a forest, the body of the real George Shaw is found. To buy time, Sawyer feigns concern that David is a virgin, and that she does not want to be his first. She convinces David to have sex with another woman and suggests Violet, who previously threatened Sawyer with a shank, and he brings her to the solitary confinement cell. Sawyer uses Violet's shank to stab David in the neck and flees as he kills Violet. He recaptures Sawyer outside, and she wakes up in the trunk of his car next to her mother's corpse. Jumping from the moving car, Sawyer flees into the woods. David catches up and breaks her ankle with a hammer. Sawyer stabs him in the eye with Angela's cross and slashes his throat with the shank. Meanwhile, it is revealed that Nate was an undercover investigative journalist sent to investigate Highland Creek. Police execute a warrant on the center and arrest the hospital administrator. Six months later, while having lunch, Sawyer sees David sitting nearby. She approaches with a knife, but upon realizing it is not him, she drops the knife and runs away.

Quarantined in a Tokyo mental hospital, a psychology professor named Kenji Murai is visited by a group of doctors asking him about the events that led him there. Murai proceeds to explain how, despite only two of his party being dead, he was the only one to be rescued. He then relates the story of his band of day trippers on a yacht: Murai, wealthy industrialist Masafumi Kasai (the owner of the yacht), salaryman skipper Naoyuki Sakuda, his shipmate assistant Senzō Koyama, celebrity writer Etsurō Yoshida, professional singer Mami Sekiguchi, and student Akiko Sōma. A sudden storm causes the yacht to nearly capsize. Though the boat remains upright, it sustains severe damage during the storm and drifts uncontrollably. The group arrive at a seemingly deserted island and begin to explore. They come across ponds full of fresh rainwater and a forest populated by unusually-large mushrooms. As they cross the island, they come upon a wrecked ship on the shore whose sails are rotted and its interior is covered with a mysterious mold. Murai, after reading the ship's log, warns them not to eat the mushrooms because they might be poisonous since the former crew had hallucinations after eating them. Finding that the mold is killed by cleaning products, they work to clear it from the ship. In doing so, they begin to suspect that the ship was connected to nuclear tests conducted in the vicinity of the island, with the resultant fallout forcing a bizarre mutation on various organisms native to the surrounding area, including the mushrooms. As the days pass, the group grows restless as their supply of food stores starts to run low. Kasai refuses to help find a way off the island and insists on living in the captain's quarters alone. One night, as Kasai is raiding the food stores, he is attacked by a grotesque-looking man who promptly disappears after encountering the group. A drunk Yoshida decides to try eating the mushrooms for their hallucinogenic properties. After scuffling with Koyama over Mami, Yoshida pulls a gun and declares his intent to have his way with the women after murdering the others (accepting that if the mushrooms do turn him into a monster, then there will be no consequences for his actions). Subdued by the others, Yoshida is locked in the captain's quarters, ironically ousting Kasai. Kasai tries to convince Naoyuki to abscond together with the food and repaired yacht. Naoyuki violently rebukes this notion, but an unstated amount of time later hogties Kasai and flees with all the gathered food (including Koyama's secret stash that they had been hoarding to extort money from Kasai). Faced with this dire prospect, Mami frees Yoshida and they attempt to take over the ship, shooting and killing Senzō in the process. Murai and Kasai manage to take the gun from Yoshida and force the two off the ship. Some time later, Kasai is confronted by Mami, who entices him to follow her into the forest and eat the mushrooms. Perpetual rainfall has caused wild fungal growth, and Kasai realizes that those who have been eating the mushrooms have turned into humanoid mushroom creatures themselves. The mushrooms are delicious and cannot be resisted after the first bite. Kasai eats the mushrooms, hallucinates scenes of Tokyo nightlife, and falls to his knees amongst the creatures. Murai finds the yacht adrift and swims out towards it. He finds a note left behind by Naoyuki listing the names of those on the island as dead and how, having now run out of food and energy, he has decided to jump into the sea. Murai draws a large X over the note. Others who have turned into mushroom creatures attack Akiko and Murai. They are separated and Akiko is kidnapped. As Murai tracks her down, he discovers that she has been fed mushrooms and is under their influence along with Mami, Yoshida, and Kasai. Murai attempts to rescue Akiko, but he is overwhelmed by the mushroom creatures and flees without her, making his way onto the yacht and escaping the island. Several days pass later, Murai is finally rescued. As he waits in the hospital, he begins to wonder if he should have stayed with Akiko on the island. His face is revealed to show signs of being infected with fungal growths. Murai states after that it did not matter whether he stayed or not, but he would have been happier there with Akiko. The screen fades as Murai notes that humans are not much different from the mushroom creatures.

Thomas de Frémont, a child prodigy obsessed with tinkering and action films, lives in a secluded and high-tech castle with his widowed mother, Julie, his diabetic and partially blind grandfather, Papy, and his pet dog, J.R. On Christmas Eve, Thomas uses the Minitel to try and communicate with Santa Claus and unknowingly makes contact with a derelict who is using a public Minitel terminal. The deranged man claims to be Santa and attempts to get Thomas to divulge his address; before their connection is severed, the vagrant learns Thomas's mother manages a nearby Printemps. While Thomas sets up a security system to record or capture Santa, the vagabond gets a job as a Santa at the Printemps but is fired from it after Julie witnesses him slap a child who accused him of not being the real Santa. The vagrant subsequently steals a Santa suit, paints his hair and beard white, and hitches a ride to Julie's home in the back of a delivery van, the driver of which he kills upon reaching the de Frémont residence. The man then murders Julie's groundskeeper and her chef, breaks into her home through the chimney, and stabs J.R. to death in front of Thomas, who is convinced the intruder is an enraged Santa. What follows is a game of cat and mouse as Thomas uses his security system and booby traps, as well as an arsenal of makeshift weaponry, to defend his enfeebled grandfather and combat the trespasser, who has cut the telephone lines and trashed the only car; the man at one point catches Thomas but then lets him go while declaring, "I win. You lose. Now I hide and you seek. Okay?" Julie, concerned over her calls home not getting through, phones the police, who send an officer to the castle to check on Thomas and Papy. The vagrant murders the policeman and recaptures Thomas, but the boy is saved when his grandfather shoots his assailant with the dead officer's gun, with Julie arriving home seconds later to find a stunned Thomas standing over the killer's body, stammering, "It's my fault, Mom. I wanted to see Santa Claus."

Since the beginning of time, there have been " Others " – humans endowed with supernatural abilities – and for just as long, the Others have been divided between the forces of Light and Dark. In Medieval times, the armies of both sides met by chance, and a great battle began. Seeing that neither side had a clear advantage, the two faction leaders, Geser and Zavulon, called a truce and each side commissioned a police -like force to ensure it was kept; the Light side's force was called the Night Watch and the Dark side's force was called the Day Watch. In 1992 Moscow, Anton Gorodetsky (Russian: Антон Городецкий) visits a witch named Daria and asks her to cast a spell to return his wife to him, agreeing that she should miscarry her illegitimate child as part of it. Just as the spell is about to be completed, two figures burst in and restrain Daria, preventing her from completing the spell. When they notice that Anton is able to see them, they realize that he is also an Other. Twelve years later, Anton has enlisted in the Night Watch. While policing Moscow, he encounters several portents that Geser says are linked to an ancient prophecy of an immensely powerful Other that will end the stalemate between Light and Dark, but will be more likely to join the Dark. Anton's investigations lead him to a nurse, Svetlana, whom disaster seems to follow everywhere, and a young boy named Yegor. In the film's climax, Anton prevents a catastrophic storm from leveling Moscow, when he realizes that Svetlana is an Other, and begins teaching her to control her power. But in the process, Anton realizes that Yegor is his own son, and that his wife was pregnant with him when Anton tried to have a spell cast on her (believing, mistakenly, that the father of the child was his wife's lover, not himself). Learning that his own father tried to kill him before he was born turns Yegor – the Other of the prophecy – against Anton and towards Zavulon, which was the latter's plan all along. In helpless rage, Anton strikes Zavulon, while saying in voice over that, although the prophecy has come true and the Dark's victory seems inevitable, he will not give up.

After the town of Kaihoro's entire population disappear, the Astro Investigation and Defence Service (AIDS) sends their agents Derek, Frank, Ozzy, and Barry to investigate. They find the town overrun by man-eating aliens disguised as humans in blue shirts. Barry kills one of the aliens and is attacked by others. After Derek notifies Frank and Ozzy, he begins torturing Robert, an alien they caught earlier. Robert's screaming attracts a number of aliens. Derek kills the would-be rescuers, but he is attacked by Robert and falls off a cliff ledge, to his presumed death. Meanwhile, charity collector Giles passes through Kaihoro. He escapes Robert's attack and comes to nearby house for help, only to get captured by another alien. Giles later wakes up in a tub of water filled with vegetables and is told he is about to be cooked and eaten. Derek wakes up in a seagull 's nest with a hole in his head, so he uses a hat to keep his brains from leaking out. That night, Frank, Ozzy, and Barry infiltrate the aliens' house and find a room filled with bloody cardboard boxes, likely containing the corpses of former Kaihoro residents. They kill an alien, and Frank wears its shirt to infiltrate their meeting. He learns that the victims have been harvested for food and hears of their recent hostage, Giles. The alien later dines on Robert's vomit, and the disguised (and disgusted) Frank has to follow suit. He escapes and tells the others of the plan. They save Giles as the aliens sleep. At sunrise, they try to leave but are attacked by the aliens, and a gunfight ensues. Derek emerges and joins the fight, using his belt as a headband when the hat is shot off. He grabs a chainsaw from the boot of his car and heads for the aliens' house. As the group leaves with Giles, the alien leader (Lord Crumb) and his followers morph into their true form and follow. Ozzy uses a rocket launcher to explode Frank's car, which has been overrun by aliens. Frank and Ozzy hunt for Lord Crumb and kill many aliens along the way. Meanwhile, Derek kills an alien with his chainsaw and replaces the missing parts of his brain with its brain. An alien prepares to shoot Frank and Ozzy, but it is beheaded by Derek. Frank and Ozzy are shocked to see him alive, and see he has lost his mind. After they escape the house, Lord Crumb shoots Ozzy in the leg and Frank fires his rocket launcher at the leader, but misses. Lord Crumb knocks out Derek and the house transforms into a giant spaceship and blasts off into space. On board, Derek looks out the window to see Earth. He ambushes Crumb and kills the alien with his chainsaw. He proclaims war on the alien, then puts on the alien leader's skin and laughs maniacally as he rockets towards the alien planet. Back on Earth, the survivors drive away in Derek's car.

After a spectacular and mysterious cosmic event, ants of different species undergo rapid evolution, develop a cross-species hive mind, and build seven strange towers with geometrically perfect designs in the Arizona desert. Except for one family, the local human population flees the strangely acting ants. Scientists James R. Lesko and Ernest D. Hubbs set up a computerized lab in a sealed dome located in an area of significant ant activity in Arizona. The ant colony and the scientific team fight each other, though the ants are the more effective aggressors. The narrative uses the scientific team as the main protagonists, but there are also ant protagonists going about their duties in the colony. The ants immunize themselves to the humans' chemical weapons and soon infiltrate their lab. Teams of ants penetrate the computers of the lab and short them out. After Lesko decodes an ant message, Kendra Eldridge (a young woman who has taken refuge with the scientists), becomes convinced that her actions have enraged the ants. Seeking to save the two scientists, she abandons the lab and apparently sacrifices herself. Hubbs and Lesko begin to have different plans for dealing with the ants. While Lesko thinks he can communicate with the ants by means of messages written in mathematics, Hubbs plans to wipe out a hill he believes to be the ants' central hive. Delirious from a venomous ant sting, Hubbs can barely get his boots on, but is determined to attack the hive and kill the ant queen. Instead, Hubbs literally falls into a trap – a deep pit that the ants fill with earth. Helpless to save Hubbs and convinced that the ants will soon move into desert areas where their growth will exceed man's ability to control them, Lesko chooses to follow Hubbs's plan. He sets out to the hive with a canister of insecticide. Descending into the hive, Lesko hunts for the queen, but instead finds Kendra reaching out from under the sand. The two embrace and Lesko realizes that, far from destroying the human race, the ants' plan is to adapt the human race and make them a part of the ants' world. In a voice-over, Lesko states that he and Kendra do not know what plans the ants have, but they are awaiting instructions.