The story concerns Ambrosio - a pious, well-respected monk in Spain - and his violent downfall. He is undone by carnal lust for his pupil, a woman disguised as a monk (Matilda), who tempts him to transgress, and, once satisfied by her, is overcome with desire for the innocent Antonia. Using magic spells, Matilda aids him in seducing Antonia, whom he later rapes and kills. Matilda is eventually revealed as an instrument of Satan in female form, who has orchestrated Ambrosio's downfall from the start. In the middle of telling this story Lewis frequently makes further digressions, which serve to heighten the Gothic atmosphere of the tale while doing little to move along the main plot. A lengthy story about a "Bleeding Nun" is told, and many incidental verses are introduced. A second romance, between Lorenzo and Antonia, also gives way to a tale of Lorenzo's sister being tortured by hypocritical nuns (as a result of a third romantic plot). Eventually, the story catches back up with Ambrosio, and in several pages of impassioned prose, Ambrosio is delivered into the hands of the Inquisition; he escapes by selling his soul to the devil for his deliverance from the death sentence which awaits him. The story ends with the devil preventing Ambrosio's attempted final repentance, and the sinful monk's prolonged torturous death. Ambrosio finds out by the devil that the woman that he had raped and killed, Antonia, was indeed his sister.