After setting his camera in search of the Eiffel Tower through the opening credits, Truffaut sets us down inside a classroom. As the students take a test, they pass around a picture of a pin-up model. Antoine Doinel draws a mustache on the woman and is caught with it, and as punishment is sent to the corner without recess privileges. It is in his nature to rebel, and so he writes a poem on the wall about the fact that he has been unfairly punished. Following recess, the class returns and their teacher recites the poem, “The Hare,” asking everyone to copy it down. Antoine, however, must stay in the corner and clean his poem off the wall.
At home, Antoine takes money hidden in the mantle, sets the table for dinner, gets flour from the store for his mother and takes out the garbage. His mother is especially hard on him, and his father—who is actually his stepfather—shows him inconsistent affection.
His parents don’t set the alarm the next morning, so Antoine is late for school. On his way, he runs into his best friend, René, and the two boys decide to skip school. On their day of hooky, Antoine and René go to the movies and play games, and at the end of the day Antoine spots his mother kissing another man, and she sees him. They do not talk about it.
At home later that night, Antoine and his father cook dinner together as Mrs. Doinel is working late. When Antoine's mother arrives late that night, she and Mr. Doinel get into a contentious argument.
When Antoine returns to school the next day he tells his teacher that his mother has died in order to get out of being punished for skipping. The lie works, until his parents arrive at school. His dad slaps him in front of his class, and Antoine decides to never return home again. He sends a note to his parents outlining that he has run away and sleeps in a printing plant that René shows him that night.
The next day, Mrs. Doinel picks Antoine up from school. She bathes him and offers him an uncharacteristic amount of love and warmth. As she puts him to bed, Antoine's mother makes a deal with him that if he is in the top 5 for his next essay, she will give him 1,000 francs.
After making the deal, Antoine becomes obsessed with Balzac, and is inspired by the writer for his next essay. He even makes a shrine to Balzac in a small box in his parent's apartment. After putting a candle in the shrine, however, he starts a small fire that almost burns the house down. Mr. Doinel scolds him, but his mother comes to his defense, and suggests that they should be less hard on him, and should all go to the movies. The family has an uncharacteristically fun evening together.
When he hands in his paper, Antoine expects to get high marks, but his composition instructor gives him an F. It turns out that Antoine just plagiarized Balzac and passed it off as his own work. Suspended from class for the rest of the term, Antoine decides that he really cannot go back to his parents' house now, so René offers him a vacant room in his own parents' house.
René and Antoine steal a bit of cash from René's mother’s stash to go to the movies, and they wind up smoking cigars and drinking wine in his bedroom. All of this turns into a plan to get money so that they can live the life they hope for, free from their parents' watchful gaze. Without any way to make money, the two boys resort to stealing. Antoine goes to Mr. Doinel's office after hours and steals his typewriter to sell.
After they cannot sell the typewriter, Antoine brings it back to the office, but is caught by a guard while putting it back. Mr. Doinel decides to turn him in to the police, and have him sent off to a state home for children.
Mrs. Doinel meets with the judge, and she is willing to let her son be taken to a juvenile observation center. Here, she reveals that Mr. Doinel is not Antoine’s biological father. At the juvenile observation center, Antoine goes to a psychologist and reveals that he knows he is not Doinel's son, and that he also knows his mother wanted to abort him during her pregnancy. It was his grandmother who stopped her from doing it. When Mrs. Doinel next visits her son, she says that she is giving up custody of him once and for all. With no one to care for him, Antoine breaks out of the observation center and makes a run for it, ending up on the beach, unsure of what to do next.
