North Carolina, the present. Linda and her husband Dr Henry Henry have a troubled relationship. He's happy playing with his elaborate model train network (which occupies the whole of the attic), but she's bored, unfulfilled and turning to drink.
Martin, a young British hitch-hiker, is picked up by a truck driver. Martin strikes up a conversation about the driver's mother-fixated tattoos, but the driver is both uninterested and faintly repelled. He eventually drops Martin off at a roadside café. There, he encounters Linda and her friend Arlanda, who are complaining about their husbands. He joins them at their table and says that he's crossed the Atlantic to look for his mother. Linda seems troubled. That night, unable to sleep, she looks out of her bedroom window and sees Martin staring at her.
Linda says that she wants to adopt a child, but Henry tells her that her past makes it impossible. He goes to work (at the geriatric clinic, where he is having an affair with Nurse Stein) and she dives into their pool and lies on the bottom. When she rises to the surface, she finds Martin standing by the edge. He knows both her married and maiden name, and starts asking her intimate questions. He tells her that he's her long-lost son, and accuses her of abandoning him. Linda tells him that she had no choice, as she was an under-age schoolgirl. He attempts to play a child's I-Spy game, but she rebuffs him.
Linda and Martin drive to a bar, where Martin tells her that he is determined to recapture the American childhood he never had. A waiter attempts to intervene, but is brusquely rebuffed by Linda. He goes over to the bartender and expresses bemusement that Linda appears to be having a conversation with herself. Linda recalls being seduced (or raped) at a fairground by a man with MOTHER tattooed across his chest: he operated the bumper cars. They go home, and Martin throws a childlike tantrum after he asks him to wash his hands. She threatens to put him to bed, and they start kissing passionately.
Henry is summoned to see Dr Bernard Fairmont, his boss, and is accused of administering the wrong drug to a patient while his mind was elsewhere. After Henry denies that domestic problems are affecting his judgement, Dr Fairmont says that Nurse Stein will have to be replaced.
While Martin draws a moustache on a framed picture and plays with her dolls, Linda complains that Henry is nice but boring. Martin says that he wants to regress back to childhood.
Henry and Nurse Stein drive to a model railway convention. She orders him to leave Linda if their relationship is to have a future. Henry's keynote speech defending his hobby drives the crowd into a frenzy, but back home Martin is systematically smashing up his trains. When the entire set-up falls through the bedroom ceiling, Linda and Martin run downstairs laughing hysterically and end up in a clinch on the floor.
Martin plays the piano to Linda, bringing her to tears. As she falls asleep, he says he's going to go upstairs and smash up the trains. She is woken up by the television (playing the dramatic climax of the 1962 film Cape Fear). Unnerved, she rings Arlanda and urges her to come over. She cannot bring herself to go upstairs and assess the damage. Arlanda arrives and Linda tells her about what happened on her 15th birthday at the travelling fair.
Linda urges her to go upstairs and see how badly the trains have been damaged, but Henry returns just as she passes the front door. Arlanda tells him how concerned she is about Linda's well-being, but Henry puts it down to her lack of interest in model trains. He goes upstairs and finds that his trains are in full working order. Calling for Linda, he opens a cupboard and a naked Martin springs out and stabs him to death. Linda packs a bag and drives off. Blood seeps through the living room ceiling. Henry can still be heard calling for Linda.
