Nathalie is the name a Parisian prostitute assumes for a special mission or "private investigation." She is engaged in this unusual and secretive task by a professional, upper-middle-class wife who fears that her husband is unfaithful to her. Nathalie has to seduce the clueless husband and regularly report all details of her relationship with him, including his most intimate sexual preferences in bed. Nathalie is stunning, charming, and cunning. Can Nathalie and her reports to the mistrustful wife be trusted? Is the middle aged husband indeed unfaithful?
In Paris, the executive Bernard and his wife, the gynecologist Catherine, form a bored upper-middle-class, middle-aged bourgeois couple. They have been married for many years and their life no longer has any sexual. One day, Catherine listens to the message on his cell phone, and she believes that Bernard is unfaithful to her. Catherine hires the beautiful prostitute Marlène to pretend to be a young woman named Nathalie, and seduce and investigate the sex life of Bernard and report his secrets and performance between walls to her. Catherine discloses a surprising revelation in the end.
- A Parisian couple, Bernard (Gerard Depardieu) and his wife Catherine (Fanny Ardant) are an upper middle-class middle aged bourgeois man and wife whose sex life has lost its excitement. Fanny plans a surprise birthday party for Bernard for his return from an overseas flight but he misses the flight and Fanny is left to celebrate alone with all the other guests. Fanny intercepts a phone message to Bernard from a woman referring to a sexual liaison but Bernard claims that the relationship means nothing to him. One evening Fanny wanders into a sex club and confronts a beautiful young prostitute Marlene (Emmanuelle Beart) with the proposition that she makes friends with her husband to find out what he likes, ostensibly so she can understand him better. Using the name Nathalie, Marlene goes through with the initially innocent plan, meeting Bernard at his favorite café, pretending to accidentally meet him and asking for a light for her cigarette and getting paid by Catherine. When Marlene reports that Bernard has taken her to a hotel for sex Catherine is upset but asks Marlene to continue reporting to her what the couple does during their trysts. Catherine bonds to Marlene and sees how her life and that of Bernard have developed in ways that can never be changed.