Lucien, a modest engineering student, discovers that his steady girl friend Mireille is using him to cover up her dates with Mario, an older man. Lucien wanders about the streets of Paris and meets Joëlle, a sophisticated teenager who supports herself with a job in a sleazy nightclub. It is late when they meet, and Joëlle invites Lucien into the club and demonstrates her stripping act to her one-man audience. A feeling of rapport develops as they go from place to place seeking entertainment; in the morning the two discover they have fallen in love. Mireille learns of the romance and, on a bet, plots with Philippe and other friends to break up the affair. Neither Lucien nor Joëlle, however, succumbs to the propositions of Mireille and Philippe. Later at a nightclub, Mario gives the appearance of seducing Joëlle. Enraged, Lucien rushes at him and they fight. Lucien is reunited with Joëlle--he has learned of the complexities of love, and she has acquired from him a sense of stability.