The philosopher Denis Diderot, one of the modernists of the French 18th-century Age of Enlightenment movement, is a guest at the château of the Baron d'Holbach. The film depicts the Baron (in reality a major sponsor of Diderot) as a benign host and inventor of amusing machines, including a piganino. Diderot's banned Encyclopédie is being secretly set up, printed and bound in a crypt beneath the chapel, where the noise of the presses is masked by the playing of an organ.
Madame Therbouche, a purported portrait painter and intrigante, arrives from Berlin. She has made a painting of Diderot's idol, Voltaire, and convinces him to be more daring and to pose for her in the nude, leading to an animated row with his wife Antoinette. This unsavoury event is witnessed by a feared visitor, Baron d'Holbach's brother, the Cardinal, who is hunting for the illegal Encyclopédie printers. To divert him from entering the chapel, the Baroness confesses to him her real and imagined sins and sends every woman in the castle to do the same. Most notably her guest, the depraved Madame de Jerfeuil, is having a lesbian affair with her cousin. The couple is later joined spontaneously by her husband the Chevalier de Jerfeuil, who has been seduced by two other male guests.
Diderot's romantic adventures oblige him to constantly revise his article on the subject of morality. Eventually, secret agenda are disclosed and the painter is revealed as a spy in the Cardinal's service. However, she finally defects to Diderot's material and sensual causes, and the Cardinal fails to find the clandestine publishing operation which has been transplanted to Amsterdam.
