Antoine Doinel (Jean-Pierre Léaud) is a young boy growing up in Paris during the 1950s. Misunderstood by his parents for playing truant from school and stealing, and tormented in school for discipline problems by his teacher (Guy Decomble), (Antoine falsely explains his being away from school was due to his mother's death), Antoine frequently runs away from both places. The boy finally quits school after being caught plagiarizing Balzac by his teacher. He steals a typewriter from his stepfather's (Albert Remy) work place to finance his plans to leave home, but is apprehended while trying to return it.
The stepfather turns Antoine over to the police and Antoine spends the night in jail, sharing a cell with prostitutes and thieves. During an interview with the judge, Antoine’s mother confesses that her husband is not Antoine’s biological father. Antoine is placed in an observation center for troubled youths near the seashore (as per his mother's wishes). A psychologist at the center probes reasons for Antoine's unhappiness, which the youth reveals in a fragmented series of monologues.
One day, while playing football with the other boys, Antoine escapes under a fence and runs away to the ocean, a place he has wanted to visit his entire life. He reaches the shoreline of the sea and runs into it. The film concludes with a freeze-frame of Antoine, and the camera optically zooms in on his face, looking into the camera.