In 1798, a feral boy is discovered outside the town of Aveyron, France. Diagnosed as mentally impaired, he is relegated to an asylum. A young doctor named Jean Itard becomes convinced that the boy has normal mental capacity, but that his development was hindered by lack of contact with society. He brings the boy home and begins an arduous attempt at education over several years.

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.