Sister Jeanne, the abbess of the Ursuline convent in Loudun, is having night visions in which Father Grandier, the Vicar of St. Peter's Church, appears wearing a heretic shirt with a rope around his neck as he is escorted by two guards. This dreadful image of the parish priest is followed by another one; in this instance, however, Grandier rests in the arms of a woman. These macabre visions let Sister Jeanne, among other convent nuns, to the belief that they had been possessed by the devil, and thus to eventually confesses her visions to Father Mignon. In the meantime, Father Grandier's unchaste life is disclosed through his relationships with Ninon (a widow), and Philippe (a young woman). Furthermore, Grandier assures his fatal destiny by opposing the decree to demolish the city's fortifications put forth by the King and the powerful Cardinal Richelieu. The act comes to a close with the exorcism of Sister Jeanne performed by Barré, the Vicar of Chinon. Barré attempts to ask her "possessor," who Jeanne calls Asmodeus: "how did you gain entry to the body of this nun?" Jeanne responds, in a man's voice: "through the offices of a friend." Jeanne states that the friend is Grandier.