Stéphane Audran plays the title character, Hélène Desvallées, the bored wife of insurance executive Charles Desvallées (Michel Bouquet). Charles suspects Hélène of playing the field, so he has a private detective locate his wife's lover, author Victor Pegala (Maurice Ronet).
Confronting Victor, Charles tries to adopt an air of indifference, but the conversation ends with the husband bludgeoning the author to death and then calmly disposing of the evidence. When Hélène is questioned about Victor's murder, she discovers on her own that her husband is guilty.
Instead of turning him in, Hélène is so thrilled that Charles cares so deeply about her that she is more in love with him than ever before. The Unfaithful Wife was directed by Claude Chabrol, the then real-life husband of leading lady Stéphane Audran.