Various experiences of childhood are seen in several sequences that take place in the small town of Thiers, France. Vignettes include a boy's awakening interest in girls, couples double-dating at the movies, brothers giving their friend a haircut, a boy dealing with an abusive home life, a baby and a cat sitting by an open window, a child telling a dirty joke, and a boy who develops a crush on his friend's mother.

At the beginning of the 20th century, Claude Roc, a young middle-class Frenchman, befriends Ann, an Englishwoman. While spending time in England with Ann’s family, Claude falls in love with her sister Muriel, but both families lay down a year-long separation without contact before they may marry.

The third in a series of films featuring François Truffaut's alter-ego, Antoine Doinel, the story resumes with Antoine being discharged from military service. His sweetheart Christine's father lands Antoine a job as a security guard, which he promptly loses. Stumbling into a position assisting a private detective, Antoine falls for his employers' seductive wife, Fabienne, and finds that he must choose between the older woman and Christine.