Holly Sargis narrates the film as a 15-year-old living in Fort Dupree, a dead-end South Dakota town. She has a strained relationship with her father, a sign painter, since her mother's death from pneumonia years earlier. Holly meets Kit Carruthers, a 25-year-old garbage collector, troubled greaser and Korean War veteran. He resembles James Dean, an actor whom Holly admires. After Kit charms Holly, he takes her virginity. As they become closer, his violent and antisocial tendencies are gradually revealed.
Holly's father disapproves of Kit and kills her dog as punishment for seeing him. Kit breaks into Holly's house and insists she run away with him. When her father threatens to call the police, Kit fatally shoots him. After Kit and Holly fake suicide by burning the house, they head for the badlands of Montana. They build a tree house in a remote area and fish and steal chickens for food. They flee when found by three men (who Kit later tells Holly were bounty hunters) whom Kit shoots dead. They seek refuge with Kit's former co-worker, Cato, but when he attempts to summon help, Kit shoots him, too. A young couple arrive and are forced into the storm cellar. Kit shoots into the closed cellar door and leaves without knowing if they are dead.
Law enforcement pursue Kit and Holly across the Midwest. They stop at a rich man's mansion and take supplies, clothing, and his Cadillac, sparing the man and his deaf housemaid. As they drive across Montana to Saskatchewan, the police find and chase them.
Holly, who has grown tired of Kit and life on the run, refuses to go with him and turns herself in. Kit leads the police on a car chase, but is soon caught. He charms the arresting officers and National Guard troops, tossing them his personal belongings as souvenirs of his crime spree. Holly reveals at the end that she received probation and married her defense attorney's son. Kit was executed for his crimes.
