Séverine Serizy is a beautiful young housewife married to a successful but emotionally distant doctor, Pierre. Despite their loving relationship, Séverine struggles with a lack of intimacy and becomes consumed by bizarre masochistic fantasies. While expressing a polite, composed exterior, she is plagued by a need to explore her hidden desires. Over time, she learns about a high-class brothel operating in Paris, where bored housewives can engage in secret, illicit activities during the day. Intrigued yet apprehensive, Séverine's curiosity gets the better of her, and she decides to visit the brothel. She meets Madame Anaïs, the proprietor, who provides her with a new identity—Belle de Jour. There, Séverine begins a clandestine double life, exploring her repressed desires while maintaining her daytime persona as Pierre's dutiful wife.

Over time, Séverine becomes enmeshed in the world of the brothel, encountering a variety of clients with peculiar requests. While she successfully keeps her activities hidden from her husband, this double life brings unexpected complications and moral dilemmas. Through her escapades, Séverine discovers more about her own sexuality and the boundaries she is willing to cross, leading her to confront the nature of her desires and the consequences they bear on her marriage and self-identity. The tension between her two worlds intensifies, pushing Séverine toward a reckoning that will ultimately challenge the facade she has so carefully constructed.