It's the late 18th century and the infamous French author, the Marquis de Sade (GEOFFREY RUSH), is imprisoned in a mental asylum overseen by Abbé Coulmier (JOAQUIN PHOENIX), a priest who's befriended the Marquis and extended him certain privileges not afforded to the other inmates, who include Cleante (MICHAEL JENN), who believes himself to be a bird; Pitou (DANNY BABINGTON) who grooms his imaginary locks; Dauphin (GEORGE YIASOUMI), a pyromaniac; and Bouchon (STEPHEN MARCUS), a hulking fellow who finds the Marquis' work highly arousing.
Despite his incarceration and isolation from his wife, Renee Pelagie (JANE MENELAUS), the Marquis continues to write and publish his erotic and sadistic works, thanks to the help of Madeleine LeClerc (KATE WINSLET), a virginal laundress who works with her mother (BILLIE WHITELAW), and enjoys the Marquis' work and helps smuggle it out of the asylum. His latest effort, "Justine," has made its way to Napoleon's attention and the emperor wants all copies of it burned and the author shot.
His followers don't do that, but they do send Dr. Royer-Collard (MICHAEL CAINE), a practitioner of torture-based rehabilitation, to perform his magic on the Marquis. Collecting the young bride, Simone (AMELIA WARNER), promised to him that he's never met, the doctor arrives at the asylum prepared to observe the Marquis before deciding what approach to take in attempting to cure him.
As Royer-Collard introduces his wife to the bedroom duties of marriage and gets a young architect, Prouix (STEPHEN MOYER), to refurbish a dilapidated estate for them, the Marquis stages a farce based on the rumors of their sex life. Despite Coulmier's objections that the theater and other literary efforts are therapeutic for the inmates, Royer-Collard orders them to be suspended.
Deprived of his ink and quills, the Marquis then sets out to continue writing, becoming ever more resourceful the more he's deprived of writing utensils as well as the belongings from his cell. From that point on, and as Madeleine gets into trouble for her relationship with the author when Charlotte (ELIZABETH BERRINGTON), a prim maid, turns her in, the Marquis continues to tell his erotic and sadistic tales any way he can, much to the dismay of both Coulmier and Royer-Collard.