The film opens with the image of a seemingly drunken young man named Jim Stark, lying down and playing with a toy monkey. He is taken in by police to the local juvenile hall, where two other teenagers have been arrested. The first, Judy, tells the policeman she goes wandering at night because she hates her father. The second, Plato, has lashed out by killing animals, due to his parents' absence. Jim offers Plato his jacket, but Plato refuses. Jim's parents and grandmother arrive and admonish him for his conduct, although Jim's father attempts to diminish the consequences of his behavior to the police. Jim finds Judy's compact, which she left behind, and keeps it. After listening to his family bicker and argue, Jim cries out that they are tearing him apart. A policeman named Ray gets Jim to confess that he resents his father for not standing up to his mother, and that Jim is afraid of being labeled "chicken." He tells Jim to come and talk to him whenever Jim is feeling lost and confused.
On the morning of his first day at school, Jim attempts to strike up a conversation with Judy on the sidewalk. Although they recognize each other from juvenile hall, she coldly brushes him off and rides to school with her boyfriend Buzz and the rest of their "gang," who openly mock Jim. On a field trip to the observatory, Jim, Plato, Judy, and the others sit in the darkened planetarium and listen to a lecture on the nature of the universe. It ends with an explosion of light and sound, simulating the end of the galaxy. Afterward, Jim comforts Plato, who has hidden under his seat, and as the students file out, the gang pressures Buzz to antagonize Jim. Buzz reluctantly agrees. Plato tries warning Jim about the gang's ambush, and suggests they go to a nearby abandoned mansion, but Buzz and the rest of the gang slash Jim's tires so he cannot leave the planetarium. Buzz then successfully goads Jim into a knife fight by calling him a "chicken." Buzz and Jim fight, until Jim ultimately knocks Buzz's knife out of his hand and pins him against the wall. Humiliated and intrigued by Jim, Buzz challenges him to a "chickie-run," a kind of race with which Jim pretends to be familiar, to be held at a nearby bluff.
Neither wanting to be labeled a "chicken" nor participate in meaningless, violent rituals, Jim asks his father for advice later that day, but he is unable to provide Jim with an adequate answer. Jim is openly disgusted by the fact that his father wears an apron and tidies up after his mother, and flees his house. At Judy's house, Judy's father slaps her when she tries to kiss him, causing her to run out of her house as well. Later that night at the bluff, Jim, Judy, Plato, Buzz, the gang, and a crowd of onlookers have gathered to watch the chickie-run. Jim and Buzz admit to one another that they actually like each other, but feel compelled to compete as enemies due to the social rules of their peer group. Plato lies to Judy that Jim is his "best friend," and likes being called Jamie. Judy gives the signal for the chickie-run, in which Jim and Buzz will barrel toward the edge of the bluff in stolen cars, with the first person to bail out being labeled a "chicken." Although Jim is able to bail out, Buzz's sleeve gets caught in the door, sending him plunging to his death on the rocks below. Judy looks down after him, pondering whether to jump, but Jim leads her away from the scene, as the rest of the gang flees.
Jim drops Judy off, and gives her her compact mirror back. Jim then parts ways with Plato, who tells Jim he wishes Jim were his father. At the Stark residence, Jim begins to doze off when his alarmed parents question him about his whereabouts. Jim tells them he was complicit in the car accident they have already heard about on television, and once again asks his father for advice about what to do. He becomes furious when his mother tells him that they are moving; to him this seems like a way for his parents to avoid dealing with the problems facing him. When his father refuses to stand up to his mother, in addition to being incapable of offering effective advice as a man, Jim throttles him on the carpet before storming out of the house. Jim finds Judy on her front lawn, and they commiserate over their chaotic home lives. Jim suggests to Judy that they go to the abandoned mansion near the planetarium that Plato mentioned earlier. Meanwhile, the gang roves the town looking for Jim, first intimidating Plato (who grabs a gun from his parents' nightstand), and then Jim's parents, out of concern that he plans to turn them into the police.
Plato arrives at the abandoned mansion to find Jim and Judy already there. Jim and Judy pretend they are newlyweds in the market for a house, as Plato leads them on a mock tour of the mansion's amenities. The three enjoy an extended moment of peace and solitude in the house, and after Plato falls asleep, Jim and Judy slip away to share a private moment by the fireplace, where Judy tells Jim she loves him. When Plato awakens, he finds the gang surrounding him, wielding a steel chain. He barely manages to escape, and in a distraught frenzy, fires his gun at Jim, and then fires at one of the gang members, injuring him. Plato escapes into the bushes around the mansion, and Jim follows him, feeling guilty for abandoning Plato. Judy tells Jim that Plato spoke of him like his father.
Plato boards himself up in the nearby planetarium, as a crowd of police and concerned onlookers gathers outside to watch the events unfold. Jim follows in after him, and manages to get Plato to give up the gun temporarily by offering up his jacket. Jim empties the chamber of the gun and hands it back to Plato. As he leads Plato out, Plato asks that the police lower their lights, which they do. However, when they see Plato is still armed, they flash their lights once more, causing Plato to rush forward. The police shoot and kill Plato as Jim looks helplessly on. Jim hunches over Plato and zips up the jacket on his lifeless body. Jim's father tells Jim that he will try to be strong for Jim, and Jim introduced Judy to his parents. Plato's caregiver continues to cry as the police and the others make their exit.
