Ambrosio, the eloquent priest, has the reputation of severity, which is revealed when Agnes (Lorenzo’s sister) comes with the other nuns to confession. She drops a letter, picked up by Ambrosio, which reveals her plans to run away with Raymond de las Cisternas. When Agnes confesses that she is pregnant with Raymond’s child, Ambrosio turns her over to the prioress of her abbey for punishment. Before she is led away, she curses Ambrosio.
A young novice, Rosario, is a constant companion to Ambrosio. Unexpectedly, Rosario admits that he is not a monk, but a woman (Matilda). She took the disguise so that she could be near Ambrosio. Ambrosio knows he must throw Rosario out of the monastery. She begs him not to, and vows to kill herself if he does. He relents unwillingly, but the next day he convinces her to leave of her own accord. Matilda asks Ambrosio to give her a rose to remember him by. As he picks the rose, he is bitten by a serpent and is rushed to his room where he battles death. It is predicated that he will die within three days. Rosario is deputed to be his nurse, and the next day it is discovered that Ambrosio is cured. It is proclaimed a miracle, but it is eventually revealed that Rosario sucked the poison from Ambrosio’s wound and is now dying herself. At the point of death, Rosario (Matilda) begs Ambrosio to make love to her, and he succumbs to the temptation at last.