Catherine discovers that Bernard had a one night stand. Because this is France, she doesn't react the way an American cinematic woman would, e.g. with violence, arguments, or divorce papers. No, since she is a sophisticated woman, she instead seeks to understand why her husband is a philanderer.
She won't get a straight answer from him. So, she hires a prostitute, Nathalie (Emmanuelle Béart), to seduce him and later confide his secrets to Catherine. Nathalie and Catherine gradually become close during their meetings, where Catherine talks (increasingly) dirty regarding Bernard. Can this marriage be saved, or what exactly is it that Nathalie and Catherine want from each other?
How others will see it. The typical American viewer will promptly form two strikes against Nathalie... The film is subtitled, and is slow. For a woman in her fifties, Catherine is hot, but her age is nonetheless showing. Nathalie is younger and comely, and the patient viewer does get to see skin here and there. And, there is no shortage of vulgar sex chatter. If such ying is your yang, you may find the film engaging. But if you are easily offended or bored, or house curious children, or anticipate hot lesbian sex involving Nathalie, you will likely be disappointed.
How I felt about it. It doesn't take long for the viewer to realize that Nathalie is bisexual. She gives Catherine plenty of steamy glances, although Catherine appears oblivious to this, or at least pretends to be. Their relationship evolves into an intimate friendship, yet it isn't clear whether Nathalie is in it strictly for the money, or what useful information Catherine hopes to learn about her straying husband.
We know that Catherine is paying Nathalie to talk dirty about Bernard. We wonder whether this talk stimulates Catherine. Or, whether it merely provides a pretext for her to continue to see Nathalie. We also wonder whether Nathalie's experience as a prostitute no longer allows her to feel or exhibit real emotions regarding sexual attraction, since lying and acting are integral to her profession. A further skill she has learned is what will please her clients, because the goal is to keep taking Catherine's money. Sex talk, friendship, and romantic intimacy are merely means to the end.