A woman of a powerful character and a priest create a religious sect, through a nun who practices automatic writing dictated by the Virgin Mary.

The story is inspired by a real event that happened a few years earlier in the German-speaking part of Switzerland. A preacher (Michel Bouquet) and his assistant-companion (Ingrid Thulin) create in rural Germany a sect they call "The Holy Family" : its members will embark on a new Ark at the Apocalypse. Proccupied, the church reacts strongly to this heresy and the couple is forced to leave for Switzerland. They find refuge in a village and start again, preaching to credulous peasants and borrowing large sums of money. In order to cover their tracks, they manage to convince the villagers that a young girl, the daughter of a rich farmer, is inhabited by the devil and that she must be exorcised. In the macabre rite that ensues, the cult members beat her to death.