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.

Michel Jussieu, a film producer, returns one morning to the cabaret where he forgot his sweater. He discovers the corpse of an extreme right-wing journalist, but immediately he is knocked out. He wakes up in the police station ...