Middle-aged, simple-minded Chance lives in a wealthy old man's townhouse in Washington, D.C., along with the man's African-American maid Louise, who is kind to him. He has spent his whole life tending the garden and never left the property. Other than gardening, his knowledge is derived entirely from television. When the old man dies, his lawyers order Chance out. He wanders aimlessly, discovering the outside world for the first time. An African-American youth points a knife at him; Chance ineffectually tries to click him out of existence with a TV remote control. Passing by a shop, he sees himself captured by a video camera in the shop window. Entranced, he steps backward off the sidewalk and is struck by a limousine chauffeuring Eve Rand, the glamorous and much younger wife of elderly business mogul Ben Rand. When she asks his name, she mishears "Chance, the gardener" as "Chauncey Gardiner". Eve brings Chance to their estate to be seen by Dr. Robert Allenby, who is resident there caring for Ben, who is dying from a blood disease. After checking Chauncey out, the doctor invites him to stay to keep an eye on him. Chauncey's manners are old-fashioned and courtly, and he wears expensively tailored but outmoded 1930s clothes he took from his former employer's attic. When Ben meets him, he assumes that "Chauncey" is an upper-class, highly educated businessman fallen on hard times. Ben admires him, finding him direct, wise, and insightful. Ben is also a confidant and advisor to the President of the United States, whom he introduces to Chauncey. In a discussion about the economy, Chance takes his cue from the words "stimulate growth" and talks about the changing seasons of the garden. The President misinterprets this as optimistic political advice and quotes "Chauncey Gardiner" in a speech. Chance rises to national prominence, attends important events, develops a close connection with the Soviet ambassador, and appears on a talk show during which his detailed advice about what a serious gardener should do is misunderstood as his opinion on presidential policy. Louise tells other African Americans as they watch Chance on TV that he has "rice pudding between the ears" and that whiteness is all that is needed to succeed in America. The President is shown as sexually impotent with his wife when watching the show. Though Chance has risen to the pinnacle of Washington society, the Secret Service and 16 foreign agencies are unable to find any background information on him. Meanwhile, Allenby becomes suspicious that Chance is not a wise political expert and that his mysterious identity may have a more mundane explanation. He considers telling Ben but remains silent when he realizes how happy Chance is making him in his final days. The dying Ben encourages Eve to become close to "Chauncey". She is already attracted to him and makes a sexual advance. Chance has no interest in or knowledge of sex but mimics a kissing scene from the 1968 film The Thomas Crown Affair, which happens to be on TV. When the scene ends, Chance stops suddenly, and Eve is confused. She asks what he likes, meaning sexually; he replies "I like to watch," meaning television. She is taken aback but masturbates for his voyeuristic pleasure, not noticing he has turned back to the TV and is imitating Lilias, Yoga and You on another channel. Chance is present at Ben's death and shows genuine sadness. Questioned by Allenby, he admits that he "loves Eve very much" and that he is just a gardener. When he leaves to inform Eve of Ben's death, Allenby says to himself, "I understand." While the President delivers a speech at Ben's funeral, the pallbearers hold a whispered discussion over potential replacements for the President in the next presidential term and agree on "Chauncey" as successor. Oblivious, Chance wanders off through Ben's wintry estate. He straightens out a pine sapling flattened by a fallen branch, then walks across the surface of a lake without sinking. He pauses, dips his umbrella deep into the water to the right of his path, then continues on, while the President is heard quoting Ben: "Life is a state of mind."

Patrol Agent in Charge Jeb Maynard (Bronson) is trying to identify the killers of a young Mexican boy and Maynard's friend, veteran USBP Senior Patrol Agent "Scooter" Jackson (Wilford Brimley). Maynard is the Agent in Charge of Otay Border Patrol Station (a fictional composite of the actual El Cajon and Brown Field stations), located in the hills east of San Diego. He is later helped by the boy's mother, Elena Morales (Karmin Murcelo) and a rookie Patrol Agent, Jimmy Fantes (Bruno Kirby). Hotchkiss (Harris) is an illegal alien smuggler, known by the aliens only as "the Marine." Hotchkiss works for a local fruit rancher, Carl Richards (Bert Remsen), a ranch he uses as a front for his alien-smuggling operation. Jackson, on routine road patrol, stops Hotchkiss's truck full of aliens, driven by a subordinate smuggler. The truck has boxes of tomatoes on top but the main body, accessed by rear doors, is a compartment carrying illegal aliens. When Jackson demands the driver open the back of the truck, Hotchkiss riding in back with the aliens shoots Jackson with a sawn-off shotgun at close range. The Mexican boy is badly wounded by stray buckshot, so Hotchkiss kills him with another blast and drags both bodies into the roadside bushes where he also parks Jackson's Border Patrol sedan. The F.B.I. is called to investigate, having primary jurisdiction investigating murders of any federal agents. The truck's driver, disturbed by the murders, is later killed when Hotchkiss wrecks the truck, attempting to mislead the FBI and the Border Patrol into believing the murders were related to drug smuggling. Maynard thinks Jackson was murdered by smugglers of aliens and tries to convince the F.B.I. that the marijuana in the truck was merely a ruse, but the F.B.I. does not want to believe him. The FBI seems convinced the murders are drug related, but not Jeb Maynard who doesn't believe someone would go to the trouble of removing all identifying information from the truck but carelessly leave several bags of marijuana behind. Jackson's and the Mexican boy's bodies are found by USBP Agents where Hotchkiss had hidden them. Agent Fantes discovers some fresh tomatoes near the bodies and Agent Maynard notices military style boot prints with odd markings on the soles among the footprints at the crime scene. A Patrol Agent's professional skills include "sign-cutting," knowing how to examine, analyze and interpret tracks and marks made in the ground, a skill used to track illegal border crossings, and his suspicion is aroused. Maynard has Fantes take the tomatoes to a university's agriculture department for analysis. Meanwhile Maynard and Fantes start checking the trails in the hills that the smugglers use to bring in illegal aliens and drugs. They find the same military boot prints along a trail where a USBP electronic ground sensor has been dug up and disabled. Hotchkiss, running a sophisticated and highly profitable alien smuggling operation, is a former U.S. Marine who had been trained on such equipment while in the Marine Corps. He had discovered the sensor and had disabled it on a prior smuggling run. After Jackson's funeral Maynard tells his boss, INS Commissioner Malcolm Wallace, about his suspicions. Wallace cautions him to proceed carefully, so Maynard begins his own investigation with Fantes's assistance. Maynard had found a San Diego address on a piece of paper in the murdered Mexican boy's clothes, and so goes there to discover the boy's mother, Elena Morales, working as a nanny at a well-to-do family's home. He takes her to the morgue to identify her son's remains and then asks for her help to find the killers. Elena agrees, so Maynard goes undercover posing as her cousin. They cross into Mexico and she introduces him to an alien smuggler in Tijuana who had brought her over the border when she last crossed. This smuggler is Hotchkiss's partner in Mexico. Maynard's physical features are such that he can pass for Mexican, but he does not speak Spanish fluently, and so Elena tells the smuggler that her cousin is simple minded and doesn't talk much. Elena pays the smuggler with money Maynard gave her, and they are smuggled across the border into the United States along with a group of thirty other migrants. Maynard hears the smuggler guiding them mention "the Marine who runs things" to Elena, but then the group is ambushed by bandits. Maynard and Elena escape unharmed but lose their chance to encounter the ringleader (Hotchkiss) and walk on towards the suburbs of San Diego. He drives Elena home and thanks her for helping, saying that if she stops by his office next week, he will try to help her straighten out her U.S. immigration status. Maynard goes home, cleans up and tries to sleep. Fantes stops by to tell him the agriculture report on the tomatoes indicates they were treated with a particular brand of pesticide. Fantes has checked with the local office of the U.S. Dept. of Agriculture and discovered that only a few large farms in the area use the brand. They meet later at the Border Patrol station to plan their next move. Maynard and Fantes visit the local fruit farms using that pesticide and arrive at Richards's fruit ranch. Maynard knows of Richards's reputation for exploiting illegal aliens, hiring them at poor wages to harvest his fruit. Hotchkiss is in the main ranch house when Maynard and Fantes arrive but stays inside as Richards goes out to speak to the Agents. While talking to Richards, Maynard notices the military boot prints in the dirt near the main house that he had detected earlier. Maynard and Fantes set up a surveillance of the ranch with the assistance of Border Patrol Agents Lambert and Monroe. Using binoculars, they see Hotchkiss wearing his combat boots and camouflage marine field jacket leaving the house with other smugglers including the Mexican who had guided the group that Maynard and Elena had infiltrated. Maynard has found "the Marine." Hotchkiss plans a large smuggling run soon that will bring hundreds of illegal aliens into the U.S. in one night, after which he plans to shut down operations until things cool off. Maynard deduces Hotchkiss's plans from what he witnesses in surveilling the ranch over the following days, calls all his agents together at the station and plans an operation to raid the Richards Ranch next evening. They raid the ranch after dark, capture the illegal aliens and smugglers and hold them in the ranch's main barn. At dawn, Hotchkiss arrives with the last load of aliens. As the Patrol Agents attempt to arrest him, he pulls out a MAC-10 machine-pistol and fires at them. They take cover and Hotchkiss jumps in a car and speeds away, but Maynard pursues him in a Border Patrol S.U.V. Hotchkiss attempts to lose him on an old dirt road that comes to a dead end, and then runs into the surrounding trees and bushes. Maynard draws his revolver and tracks him through the woods. Hotchkiss circles back thinking he has given Maynard the slip, but just as he is about to get back into his car, Maynard appears with his revolver pointed at Hotchkiss and says, "The end of the road." Hotchkiss wheels about and starts firing but Maynard kills Hotchkiss with a single shot. Maynard and Fantes watch Richards leaving San Diego's United States District Court House after being sentenced to two-to-five years in federal prison. Richards' boss, also on trial, is not convicted for "lack of evidence." Fantes laments that Richards will probably go right back to work smuggling aliens within a month of release. Maynard smiles and responds by saying, "It's okay kid, so will we."

Pepe the Frog, a character created by Matt Furie and first featured in a comic on MySpace called Boy's Club, is one of four twentysomething postcollegiate slacker friends who live together. In one installment, Pepe is caught by one of his housemates with his pants around his ankles, urinating. Asked why, he replies, "Feels good man". The image becomes a viral Internet meme and is co-opted by the alt-right. Furie attempts to take Pepe back from the alt-right who have turned him from a cartoon character into a symbol for hate. The film deals with the question of whether Pepe can be redeemed. The coda of the film alludes to Pepe's appropriation by pro-democracy demonstrators during the 2019–2020 Hong Kong protests.

In a post-apocalyptic realm called Anmaere, an annual drive ships wild horses from a rundown city called Whithren to another, far-off city, Levithen. Many denizens of Whithren hope to board the boat with the horses and travel to Levithen, which they believe holds a more promising future for them.