When Monsieur Husson mentions an acquaintance who works at a local brothel, Séverine becomes curious about prostitution as a means of satisfying her desires.

The situation becomes more complicated when Séverine decides to leave the brothel, with Madame Anaïs's agreement, after Marcel becomes too demanding and jealous of her husband.

The police attribute the motive to the unidentified Belle de Jour, and Séverine lives in terror of the eventual discovery, which is made almost certain by her maid's recognition of Marcel from his visit to their house.

Even though Séverine has been protected from the truth by Marcel's accomplice, Hipollyte, she ultimately decides to tell Pierre what happened.

[1] The 1967 film adaptation starred Catherine Deneuve, Jean Sorel and Michel Piccoli and was directed by Luis Buñuel.