A young woman fashion photographer, known only as O, is taken by her lover René to Château Roissy, where she is subject to various sexual and sadomasochistic acts as part of her training to serve the members of the club.
At various times, she is stripped, blindfolded, chained, whipped, made to wear increasingly large plugs to widen her anus, pierced, branded on one of her buttocks, made to wear a bird mask, and taught to be constantly available for oral, vaginal, and anal intercourse. She leaves Roissy wearing an iron ring as a sign of her initiation and indicator to men in the society that she is a sex slave.
O meets a vain model named Jacqueline, whom she photographs and grows enamored with. René introduces O to his much older step-brother, Sir Stephen, and the two men share O, as René wishes O to learn to obey and serve someone whom she does not love. While Sir Stephen proves to be a more severe and strict master than René, O soon believes he is in love with her.
At Sir Stephen's direction, O is sent to an all-female country house in Samois, run by a woman named Anne-Marie, where she undergoes further beatings and training in submission. O's visit concludes with having rings pierced into her labia at the request of Sir Stephen, and receiving a brand with his initials.
Sir Stephen tells O that René is in love with Jacqueline, and that O must seduce her and get her to undergo training at Roissy. While at first resistant to getting Jacqueline to go to Roissy, O eventually agrees. Jacqueline moves into O's flat, and is seduced by her. O reveals her BDSM lifestyle and describes her stay at Roissy to Jacqueline, who is initially repulsed and disbelieving.
Sir Stephen shares O with two other men of his acquaintance, one simply known as "the commander" and the other a young man named Ivan. After one sexual encounter with O, Ivan believes himself to be in love with her and requests Sir Stephen release her. However O refuses to leave Sir Stephen.
O takes Jacqueline to Roissy where she will be trained to serve René. Later Sir Stephen and O visit the commander's home in Brittany for a party, where O is treated as a visual spectacle, wearing nothing but chains and an owl mask. Watching O at the party, Sir Stephen feels that his ownership of her is complete.
Some time after that, O asks Sir Stephen if he would endure the same punishments and experiences that she has undergone to show her unconditional love for him. When he says "I suppose so", she suddenly burns his hand with a hot cigarette holder, leaving there a circle, or an O.
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