The Prioress of the Ursuline Convent at Loudun, Sister Jeanne, has a nocturnal vision in which Urbain Grandier, the Vicar of St. Peter’s Church, dreadfully disfigured by torture, is dragged in a hairshirt to a place of execution where he is to be burned alive. Superstition, hysteria and hopeless love for Grandier have led several of the Sisters of the convent, including Jeanne, to the belief that they are possessed by Devils. This evil possession is the primary cause which will lead to Grandier’s downfall. Other factors, however, are subsequently revealed, including his relationships with the widow, Ninon, and with the young Phillipe, who is expecting his child. Grandier further assures his doom by siding against the King and Cardinal Richelieu’s decree to demolish the city’s fortifications, which have given protection to the Protestants.

The exorcism of the Nuns begins. Spiritual and secular dignitaries attempt in vain to expel the Devil from the pious Sisters despite the objections of the Archbishop and the city fathers. Better results are expected from the rites when they are continued in public as a great spectacle for the populace. Since Grandier has supported the Mayor of Loudun in his opposition to the Paris-decreed destruction of the city’s defenses, and since his name continues to be associated with the origin of the Nuns’ bewitching, he is arrested as he enters his Church.

Grandier is found guilty of having been the source of the possession, because of his bond with the Devil, blasphemy, and his unchaste life. He is sentenced to death at the stake, but is first to be tortured into admitting his own guilt. It is finally revealed to Jeanne, during her prayers, that it is her unrequited love for Grandier which has been the real cause of her possession.