A young woman is writing a series of love letters, telling her lover the story of O, for his entertainment, but also, it seems, as a challenge. O is a fashion photographer, who falls for Rene, a wealthy man who proceeds to take her to the manor of Sir Stephen, where for several weeks, along with other women, she is enslaved in bondage and subject to torture, humiliation, and sexual assault at the hands of Sir Stephen, Rene, and their friends. After a time, Rene takes her back to her old life where she remains in love with him and has learned that when in love, "you are not free at all."