Antoine is sent to a juvenile observation center and is disowned by his parents. He manages to escape and runs to the beach. The film ends with no certainty as to Antoine's future.
This cinematic masterpiece (a semi-autobiographical film of director François Truffaut's childhood) follows the journey of a young boy named Antoine Doinel (Jean-Pierre Léaud).
Antoine attends an all-boy's school where the teachers are incredibly strict and generally abusive to the children. Antoine comes from a poor family who share a tiny two-bed apartment. His mother, Gilberte (Claire Maurier), is very strict and generally cold to Antoine. His adoptive father Julien (Albert Rémy) is kind-hearted, but generally a lax parent.
One day, while cutting class with his best friend René, Antoine sees his mother kissing another man in the street. Needing an excuse for cutting class the next day, Antoine blurts out that his mother has died. Antoine's parents, having been alerted to his truancy by one of Antoine's nosy classmates, arrive at the school to discuss the matter. When they are told that Antoine has claimed that his mother is dead, Julien becomes enraged and slaps Antoine in front of his class.
After school, Antoine runs away after leaving a note for his parents saying that he was going to make his own living and come back in the future to discuss "everything." Though he spends the night on the streets, he returns to school where his mother is waiting for him. She takes him back to the apartment and is very kind to him, especially after she realizes that Antoine never intended to reveal her affair to Julien. Gilberte begins to empathize with Antoine, noting she herself was a troubled kid once. She offers to pay Antoine $10,000 francs if he gets top marks on his next French assignment. Gilberte and Julien also make efforts to treat Antoine with greater kindness.
Inspired by his mom's offer and improved attitude, Antoine begins reading more. He becomes especially attached to the works of the author Balzac. When given an in-class writing assignment, Antoine incorporates from memory some of the lines of Balzac. The next day, the teacher publicly announces that Antoine received the lowest grade, accuses him of plagiarism and expels Antoine from school. René sticks up for Antoine and is suspended for the rest of the term. The two children decide to run away together.
It turns out that although René is rich, his parents (a mom who is an alcoholic and a dad who is a compulsive gambler) don't care about him at all. The two decide to steal a typewriter from Julien's office and pawn it for money to start a business together at the seashore. Antoine notes that he has never seen the ocean, though it has always been a dream of his. Though they successfully steal the typewriter, they are unable to pawn it and decide to return it to the office. While returning it, Antoine is caught with the machine and Julien notified. Unwilling to deal with Antoine's behavior any longer, Julien brings him to jail where Antoine stays in a cell over night.
Antoine's parents debate whether to relinquish their parental rights and make Antoine a ward of the State. In order to best assess the situation, Antoine is sent to a juvenile observation center near the seashore. Many of the kids there arein the same boat as Antoine - troublemakers, but well-meaning children. While there, Antoine admits to causing trouble and engaging in various petty crimes throughout his life. He also notes that he is aware Julien is not his real father and notes that he was abandoned by them for years to live with his grandparents. Antoine also states that he knows his mother had wanted to abort him when she was pregnant and that they have left him alone on multiple occasions to go on vacation without him.
René comes to visit, but is not allowed inside the center. Gilberte also comes to visit. She notes that her affair has been exposed and accuses Antoine of exposing the affair in a letter sent to Julien (Antoine denies doing any such thing). She tells Antoine that Julien has washed his hands of Antoine and that she is inclined to terminate her parental rights as well.
Soon after, Antoine escapes from the center during a soccer game. He is pursued, but manages to elude the authorities. Antoine continues to run until he reaches the beach and sees the wide-open ocean for the first time in his life. Antoine stands there and the film ends with a freeze-framed close-up of his face, uncertain what the future awaits him.