The middle-aged Marquis de Sade arrives at his ancestral estate of La Coste, having escaped incarceration. In the theater at the castle, he meets his uncle, the Abbe, who persuades him to stay to watch an entertainment that has been prepared for him. The play is a parody of the Marquis’ parents haggling with M. and Mme. Montreuil over the prospective marriage of their children, leading to a flashback in time to the actual negotiations.
The young Louis flees the proposed marriage to Mlle. Renée de Montreuil, but returns and marries her under threat of imprisonment. Louis would prefer Renée’s younger sister Anne, finding Renée to be very frightened and cold to his charms. At an orgy with several young prostitutes, Louis begins to get very rough in his play and explains some of his philosophy to the women, leading to the first in a long series of imprisonments.
Released into the custody of his mother-in-law Mme. de Montreuil, Louis finds himself a prisoner in his own home. When Anne is sent away to a convent school, Louis begins liaisons first with his mother-in-law’s protégé, Mlle. Collette, and then with an actress, La Beauvoisin, for whom he builds a theater at La Coste. The first play performed is for the benefit of the Abbe, who is chagrined to see that the performance is about his own misuse of the young boy Louis. In a flashback, the actual event is played out, the Marquis’s later deeds and philosophy thus being given a cold-Freudian origin.
Louis proceeds through a series of flashbacks involving his father’s death, a mysterious and recurring old man, and the baptism day of one of Louis’ own children, culminating in the scandal of Rose Keller, a widow whom he ties up and flagellates with a sword. Mme. de Montreuil is forced to pay Rose for her silence, and to send Louis back into exile at La Coste.
Louis continues to pursue Anne, and after an elaborate orgy where he is whipped into unconsciousness, he flees to Italy with the young woman. Returned to prison, Louis is tormented with visions of Mme. de Montreuil disowning Anne and his uncle the Abbe seducing her. Mme. de Montreuil visits him in prison, and tearfully tells him he has ruined her family and that he will remain imprisoned forever.
Back on the stage, a mock trial is held where the Marquis is accused of murdering Anne. The mysterious old man is present at the proceedings, and Anne herself appears to accuse Louis of her murder. Louis ruefully remembers Anne’s death in Italy from the plague. An older Louis talks with Renée about their misfortunes and regrets, telling her he can find no meaning in life. At another drunken and destructive debauchery, Louis begins to see visions of Renée in the midst of his revel. The old man lies on his death bed in prison, crying out for Renee’s forgiveness. It is revealed that the old man is the Marquis himself, following the young Marquis through his memories as he seeks his one moment of reality. Deciding to look one last time, the old man closes his eyes as the scene cuts back the middle-aged Marquis arriving at La Coste.