During the WWII American occupation of Italy, American GIs are quartered in the homes of town residents in the village of San Forino, and recently orphaned Carla "Campbell" quarters at her small home one airman at a time. The 16-year old Carla seeks comfort and sleeps with each of the three American GIs quartered alone with her in the course of 10 days: Cpl. Phil Newman, Lt. Justin Young, and Sgt. Walter Braddock. After each, in turn, moves on, Carla discovers herself pregnant. Uncertain of which is the father, Carla writes to each of the three, who are unaware of the existence of the other two, to support "his" daughter, Gia, with monthly financial payments. Over the next 20 years, Carla shrewdly invests the funds, buying a wine vineyard that she runs with the help of her handsome manager, Vittorio, and in time sends Gia to an American boarding school in Switzerland. To protect her reputation, as well as the reputation of her child, Carla has raised the girl to believe her mother is the widow of a nonexistent army captain named Eddie Campbell, a name she borrowed from a can of soup (otherwise he would have been Captain Coca-Cola, the only other term in English she knew at the time). As the widow of Captain Campbell, Carla gains social prominence in the community that would have shunned her as an unwed mother. The only individual in the community who does not pay her deference is the Contessa, a social rival. Twenty years after the end of World War II, the three ex-airmen who quartered with Carla attend a unit-wide reunion of the 293rd Squadron of the 15th Air Force in the village where they were stationed. The men are accompanied by their wives, and in the Newmans' case, three obnoxious children. Carla is forced into a series of comic situations as she tries to keep the "three fathers"—each one anxious to meet his daughter Gia for the first time—from discovering her secret. At the same time, Carla strives to convince Gia not to run off to Paris to be with a much older married man who will take her to Brazil. Vittorio, Carla's long-time companion, also must adjust to the unraveling situation, setting aside his ego to emotionally support Carla and Gia through the crisis. Meanwhile, Gia is anxious to learn more from the veterans at the reunion about her nonexistent father, Captain Eddie Campbell, while Carla tries to assure her that people's memories are short. The wives of Newman, Young, and Braddock meet Carla and Gia at the local beauty parlor. Moved by Gia's account of her father's connection to the squadron, they advocate for a memorial chapel to be named after the gallant Capt. Campbell. Eventually the "three fathers" and their wives stumble on the truth when Carla's housekeeper inadvertently informs them, mistakenly assuming that the three men who come to see Carla together have learned the truth. When confronted, Mrs. Campbell admits she does not know which of the three men is Gia's father. She challenges the men by asking them what kind of father each would have been, particularly because they have never been there for all the small but important life events of their daughter. Provoked by this, the potential fathers talk to Gia and insist that she cannot run off. Vittorio also helps Gia understand her mother's motivations for deception to protect her child and herself. At the town's dedication ceremony for the chapel, Carla says that Eddie Campbell would have been too humble to accept the honor and insists that her social rival, the Contessa, accept the dedication of the chapel in the name of the people of San Fiorino. The "fathers" cease the support payments, and the Braddocks, who cannot have children of their own, agree to have Gia stay with them while she attends college in the US. Vittorio stays on with Carla after she convinces him that he will be her sole romantic interest and business partner.

Tim Dingman and Nick Vanderpark are best friends, neighbors and co-workers at 3M. Nick is constantly coming up with crazy ideas to get rich quick, and when he invents Vapoorize, a spray that instantly disintegrates dog feces, he actually succeeds. As Nick's wealth continues to grow, so does Tim's envy, as he had initially scoffed at the idea and squandered an opportunity to invest and become mega-rich himself. Nick is blissfully unaware of Tim's envy, and his generosity only serves to make Tim more envious of him. Meanwhile, Nick's wife Natalie decides to run for the state senate but is continually plagued by questions about her husband's product. After Tim's wife Debbie and children temporarily leave and he is fired from 3M, Tim's envy reaches new levels. In a bar, he meets J-Man, a bizarre drifter, who lends a sympathetic ear and offers advice. After a drunken night out, Tim accidentally shoots Nick's beloved horse, Corky, with a bow and arrow, apparently killing him, and buries the horse in his abandoned swimming pool. Nick offers a $50,000 reward for the return of his horse. J-Man and Tim concoct a plan whereby J-Man would discover the horse and claim the reward, splitting the proceeds. However, a series of unfortunate events, including Tim's family getting holed up in J-Man's mountain cabin, leads to the horse's carcass being lost in a rainstorm. Nick reveals to Tim that he is going to Rome for the debut of Vapoorize there and gives Tim the opportunity to join him in a 50/50 partnership, which he accepts. J-Man finds out that Tim is now rich, and, feeling betrayed, tries to blackmail him. After confessing to his wife, now enjoying her rich lifestyle, Tim agrees to pay J-Man; however, J-Man ups his demands and asks to be Tim's partner. Tim accidentally shoots him in the back with an arrow and J-Man, believing that Tim has tried to kill him, backs down in fear. Tim eventually confesses all to Nick who forgives him for his jealousy and agrees to continue with the partnership; however, at a press conference for Natalie's electoral campaign (where she promises to withdraw her candidacy if it is proven Vapoorize is harmful to the environment in any way), Corky's body is seen floating down the nearby river. The animal's post-mortem reveals that the horse was not killed by the arrow as previously thought but actually poisoned by a by-product of Vapoorize, used by Tim to treat his garden after Corky came to eat the apples from his tree. The veterinarian informs the pair that she is obliged to notify the Environmental Protection Agency, and Vapoorize is immediately pulled from the market. Nick and Tim almost lose all their wealth and glory, until Tim comes up with an invention of his own: Pocket Flan, inspired by Nick's family's love for the dessert. J-Man is shown in the audience of Tim and Nick's infomercial for Pocket Flan, apparently reconciled.

Three young women—Kelly MacNamara, Casey Anderson, and Petronella "Pet" Danforth—perform in a rock band, the Kelly Affair, managed by Harris Allsworth, Kelly's boyfriend. The four travel to Los Angeles to find Kelly's estranged aunt, Susan Lake, heiress to a family fortune. After Susan promises Kelly a third of her inheritance, Porter Hall, her sleazy financial advisor, discredits Kelly as a " hippie " to dissuade Susan from dividing the fortune he secretly wants to embezzle. Undeterred, Susan introduces the Kelly Affair to a flamboyant, well-connected rock producer, Ronnie "Z-Man" Barzell, who coaxes them into an impromptu performance at one of his outrageous parties (after a set by the real-life band Strawberry Alarm Clock). The band is so well-received that Z-Man becomes their svengali manager, changing their name to the Carrie Nations and starting a long-simmering feud with Harris. Kelly drifts away from Harris and dates Lance Rocke, a high-priced gigolo, who has his own designs on her inheritance. After losing Kelly, Harris is seduced by the sexually aggressive porn star Ashley St. Ives. She soon tires of his conventional nature and waning libido due to increasing drug and alcohol intake. Harris's further descent into drug and alcohol use leads to a fistfight with Lance and a one-night stand with Casey, which results in pregnancy. Kelly ends her affair with Lance after he severely beats Harris. Casey, distraught at getting pregnant and wary of men's foibles, has a lesbian affair with fashion designer Roxanne, who pressures her to have an abortion. Pet has a seemingly enchanted romance with law student Emerson Thorne after a meet cute at one of Z-Man's parties. Their fairy-tale romance frays when Pet sleeps with Randy Black, a violent prize fighter who beats up Emerson and tries to run him down with a car. Porter offers Kelly $50,000 to relinquish any claim to Susan's inheritance. When Kelly angrily rejects his offer at one of Z-Man's parties, Susan learns of his underhanded ploy and severs her ties with him. The Carrie Nations release several records despite constant touring and drug use. Upset at being pushed to the sidelines, Harris attempts suicide by leaping from the rafters of a sound stage during a television appearance by the band. Harris survives the fall but becomes paraplegic from his injuries. Kelly devotes herself to caring for Harris, and Emerson forgives Pet for her infidelity. Casey and Roxanne share a tender romance, and Susan Lake is reunited with her former fiancé, Baxter Wolfe. This idyllic existence ends when Z-Man invites Casey, Roxanne, and Lance to a psychedelic -fueled party at his house. After Z-Man tries to seduce Lance, who spurns him, he reveals that he has breasts and is a female in drag. Z-Man goes on a murderous rampage: he kills Lance with a sword, stabs his servant Otto to death, and shoots Roxanne and Casey, killing them. Responding to a desperate phone call Casey made shortly before her death, Kelly, Harris, Pet, and Emerson arrive at Z-Man's house to subdue him. Pet is wounded in the melee, which ends in Z-Man's death. Harris is able to move his feet, which is the start of his recovery from paralysis. Three couples—Kelly and Harris, Pet and Emerson, and Susan and Baxter— wed while Porter watches from outside the courthouse window.

Joel Barish discovers that his estranged girlfriend, Clementine Kruczynski, has undergone a procedure to have her memories of him erased by the suburban Long Island firm Lacuna. Heartbroken, he decides to undergo the same procedure. In preparation, he records a tape recounting his memories of their volatile relationship. The Lacuna employees work on Joel's brain as he sleeps in his apartment so that he will wake up with no memory of the procedure. One employee, Patrick, leaves to see Clementine; since her procedure, he has been using Clementine's memories of Joel as a guide to seduce her. While the procedure runs on Joel's brain, the technician, Stan, and the secretary, Mary, take drugs, party, and have sex. Joel re-experiences his memories of Clementine in reverse chronological order as they are erased, starting with their last fight. As he reaches earlier, happier memories, he realizes that he does not want to forget her. His mental projection of Clementine suggests that he hides her in memories that do not involve her. This halts the procedure, but Stan calls his boss, Howard, who arrives and restarts it. Joel comes to his last remaining memory of Clementine: the day they first met, on a beach in Montauk. As the memory crumbles around them, Clementine tells Joel to meet her in Montauk. In Joel's apartment, Mary tries to impress Howard through their mutual interest in poetry by reciting a verse from Eloisa to Abelard. While Stan is outside, she tells Howard she is in love with him and they kiss. Howard's wife arrives and sees them through the window. Enraged, she tells Howard to tell Mary the truth: Mary and Howard had previously had an affair, and Mary had her memories of it erased. Disgusted, Mary steals the Lacuna records and mails them to the patients, including Joel and Clementine. Joel wakes up on Valentine's Day with his memories of Clementine erased. He impulsively takes the Long Island Rail Road to Montauk and calls in sick to work. He accidentally meets Clementine on the train ride home; they are drawn to one another, and go on a date to the frozen Charles River in Boston. Joel drives Clementine home and Patrick sees the two of them, realizing they have found each other again. Joel and Clementine receive their Lacuna records from Mary and listen to their tapes together. They are shocked by the bitter memories they had of each other and almost separate again, but agree to try again.