Nathalie...

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.