The philosopher Denis Diderot, one of the modernists of the French 18th century Age of Enlightenment movement, is a guest at the rural castle of the Baron d'Holbach. The film depicts the Baron (in true life a major sponsor of Diderot) as adversary and inventor of a large variety of humorous machines. The noise of a Piganino, one of the inventions, is being used to hide the printing of his forbidden Encyclopédie in the chapel, drowning the noise of the presses in Jewish (sic!) assistant Abraham's organ playing.
Madame Therbouche, a flirtatious painter and intrigante, arrives from Berlin. She has made a painting of Diderot's idol, fellow-philosopher 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. The scene is witnessed by a feared visitor, Baron d'Holbach's brother, the Cardinal, who is hunting for the illegal Encyclopédie printers. To divert him, 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 had just been seduced by two male guests of the Baron,) the Marquis de Cambrol, and the Marquis de Lutz.
Through it all, Diderot's ideas keep changing about the article he is working on, morality. Eventually a number of secrets come out: the painter's true agenda (she is a thief) and what is going on in the chapel. Meanwhile, the Cardinal has to recollect himself after hearing about too many lustful sins.