Séverine Serizy, a young and beautiful housewife, longs for physical closeness with her husband, Pierre Serizy, but their love remains unfulfilled as she cannot share intimate moments. Her sexual life is confined to elaborate fantasies involving domination, sadomasochism, and bondage, which she keeps private. Although frustrated by his wife's frigidity toward him, he respects her wishes.
During a ski trip they meet two friends, Henri Husson and Renée. Back in Paris, Séverine learns that a common friend, Henriette, now works at a brothel. At her home, Séverine receives roses from Husson and is unsettled by the gesture. At the tennis courts, she meets Husson and they discuss Henriette and houses of pleasure. Husson mentions a high-class brothel at 11 Cité Jean de Saumur. He also confesses his desire for her, but Séverine rejects his advances.
Haunted by childhood memories, including one involving a man who appears to touch her inappropriately, Séverine goes to the high-class brothel, which is run by Madame Anaïs, who names her "Belle de Jour." That afternoon Séverine services her first client. Reluctant at first, she responds to the "firm hand" of Madame Anaïs and has sex with the stranger. After staying away for a week, Séverine returns to the brothel and begins working from two to five o'clock each day, returning to her unsuspecting husband in the evenings. One day, Husson comes to visit her at home, but Séverine refuses to see him. Still, she fantasizes about having sex with him in her husband's presence. At the same time, Séverine's physical relationship with her husband is improving and she begins having sex with him.
Séverine becomes involved with a young criminal, Marcel, who offers her the kind of thrills and excitement of her fantasies. One day, Husson visits the brothel and discovers Séverine, who asks him not to tell her husband about her secret life. He agrees and rejects her services, stating he is no longer attracted to her. Séverine decides to leave the brothel, with Madame Anaïs' blessing as she believes it is due to Marcel becoming increasingly jealous and demanding. After one of his associates follows Séverine to her home, Marcel visits her and threatens to reveal her secret to her husband. Séverine pleads with him to leave, which he does, referring to her husband as "the obstacle."
Marcel waits downstairs for Pierre to return home and shoots him three times. Marcel then flees but is shot dead by police. Séverine's husband survives but is left in a coma. The police are unable to find a motive for the attempted murder. Sometime later Séverine is at home taking care of Pierre, who is now paralysed, blind and in a wheelchair. Husson visits Pierre to tell him the truth about his wife's secret life; she does not try to stop him. After Husson leaves, Séverine returns to see Pierre crying. In an ambiguous ending which is hinted to be another of her fantasies, Pierre then gets out of the wheelchair, pours himself a drink and discusses holiday plans with Séverine.
