The Monk 's protagonist, Ambrosio, who is a monk as well as a foundling of mysterious past and parentage, has risen to the position of abbot of the Capuchins, becoming a well-respected figure in medieval Madrid, revered by the populace. At the monastery, a young novitiate named Rosario approaches Ambrosio and reveals that he is actually a woman named Matilda de Villanges, whose love for Ambrosio has led her to disguise herself in order to be nearer to him. The two consummate a sexual relationship, though Ambrosio later feels remorse and disgust for his actions. After his interlude with Matilda, while visiting the nearby convent of St. Clare, Ambrosio discovers that Agnes, a nun, desires to elope with her lover, Don Raymond de las Cisternas. The monk discloses this information to Mother St. Agatha, prioress of the convent, who punishes Agnes by imprisoning her in a dungeon beneath the convent. Later, Ambrosio travels to the house of the ailing Donna Elvira Dalfa and there falls in love with her young daughter, Antonia. With the aid of Matilda and her knowledge of black magic, the monk summons a demon so that he might violate the girl. Ambrosio returns to Donna Elvira's house, kills her, and abducts Antonia, now unconscious through the action of a magical potion. In the meantime, Agnes's brother, Lorenzo, accuses Mother St. Agatha of murdering his sister and wins a warrant for her arrest. An angry mob forms in response to the accusation, and the crowd razes the convent, murdering the prioress and many innocent nuns. Amid the chaos, Lorenzo enters the convent grounds in search of his sister. When he finds her she is close to death and clutching the decaying body of her dead child. Hearing the screams of a young girl nearby, Lorenzo discovers Antonia's ravished and stabbed body and observes her attacker, Ambrosio, as he flees; later he notifies the Inquisition of Ambrosio's crimes. Ordered to be burned at the stake, Ambrosio, at the urgings of Matilda, makes a pact with Satan, exchanging his soul for freedom. The devil appears and saves him from the flames of the Inquisition, only to reveal that in killing Donna Elvira and raping Antonia, he has murdered his own mother and committed incest with his sister. The story ends as the monk's forfeit soul is cast into hell.