A film director, Jean, his producer, Marc, and his assistant, Lucette, board the Trans-Europ-Express in Paris bound for Antwerp. Once in their compartment it occurs to them that the drama of life aboard the train presents possibilities for a film, and they begin to write a script about dope smuggling. Subsequently, they see actor Jean-Louis Trintignant walking through the station. As seen through the eyes of Jean, Marc, and Lucette, Trintignant becomes Elias, the chief character in the script. Elias is going to Antwerp to pick up a suitcase of cocaine for delivery to an international organization based in Paris.

Beginning with "The 400 Blows," director Francois Truffaut made a series of films about the impetuous Antoine Doinel, in which this is the last. Antoine is now 30, working as a proofreader and getting divorced from his his wife. It being the first "no-fault" divorce in France, a media circus erupts, dredging up Antoine's past. Indecisive about his new love with a store clerk, he impulsively takes off with an old flame.