In Los Angeles, teenager Jim Stark is arrested and taken to the juvenile division of a police station for "plain drunkenness". At the station he meets John "Plato" Crawford, who was brought in for killing a litter of puppies, and Judy, who was brought in for curfew violation. The three each separately reveal their innermost frustrations to the officers; all three of them suffer from problems at home:
- Jim feels betrayed and anguished by his constantly bickering parents, Frank and Carol, but even more so by his father's timid attitude and failure to stand up to Carol. His frustrations are made manifest to officer Ray Fremick when Jim is released to their custody.
- Judy is convinced that her father ignores her because she is no longer a little girl, so she dresses up in racy clothes to get attention, which only causes her father to call her a "dirty tramp".
- Plato's father abandoned his family when he was a toddler, and his mother is often away from home, leaving Plato in the care of his housekeeper.
On the way to his first day at Dawson High, Jim again meets Judy and offers her a ride. Seemingly unimpressed by Jim at first, she declines and is instead picked up by her "friends", a gang of delinquents led by "Buzz" Gunderson. Jim is shunned by the rest of the student body but is befriended by Plato, who comes to idolize Jim as a father figure.
After a field trip to Griffith Observatory, Buzz provokes and challenges Jim to a knife fight. Jim bests Buzz in the knife fight, so to preserve his status as gang leader, Buzz suggests stealing some cars to have a "Chickie Run" at a seaside cliff. At home, Jim ambiguously asks his father for advice about defending one's honor in a dangerous situation, but Frank advises him against confrontation of any kind. That night, during the chickie run, Buzz plunges to his death when the strap on his jacket sleeve becomes entangled with his door-latch lever, preventing him from exiting the car in time. As police approach, the gang flees, leaving Judy behind, but Jim patiently persuades her to leave with him and Plato.
Jim later confides to his parents his involvement in the crash and considers turning himself in. When Carol declares they are moving again, Jim protests and pleads with Frank to stand up for him, but when Frank refuses Jim attacks him in frustration, then storms off to the police station to confess, but he is turned away by the desk sergeant. Jim drives back home, and finds Judy waiting for him. She apologizes for her prior treatment of him due to peer pressure, and the two begin to fall in love. Agreeing that they will never return to their respective homes, Jim suggests they visit an old deserted mansion Plato told him about.
Meanwhile, Plato is intercepted by three members of Buzz's gang, who are convinced that Jim betrayed them to the police. They steal Plato’s notebook and go off after Jim; Plato retrieves his mother’s gun and leaves to warn Jim and Judy, where he finds them at the mansion. The three new friends act out a fantasy as a family. Plato then falls asleep, and Jim and Judy leave to explore the mansion, where they share their first kiss. Buzz's gang find and wake up Plato who, frightened and distraught, shoots and wounds one of the gang. When Jim returns, he attempts to restrain Plato, but he flees, accusing Jim of leaving him behind.
Plato runs to the observatory and barricades himself inside as more police converge including Fremick who, with Frank and Carol, was searching for Jim. Jim and Judy follow Plato into the observatory, where Jim persuades Plato to trade the gun for his red jacket; Jim quietly removes the ammunition before returning it, and then convinces Plato to come outside. But when the police notice that Plato still has the gun they shoot Plato down as he charges them, unaware that Jim had removed the bullets. Frank comforts his grieving son, vowing to be a stronger father. Now reconciled to his parents, Jim introduces them to Judy.
