Antonia is grief-stricken at the death of her mother. She is alone, with her aunt Leonella married and distant, Raymond ill and ignorant of her plight, and Lorenzo off to procure from the Inquisition an arrest order in the death of his sister. One night she wanders into Elvira’s room and sees what she takes to be her mother’s ghost. The ghost warns her that it will return in three nights and Antonia will die. Terrified, Antonia faints and is found by her landlady, Jacintha, who goes to Ambrosio, requesting him to exorcise her home.
Under Matilda’s advice, Ambrosio prepares a concoction that will induce a condition appearing to be death for Antonia. While he is attending Antonia, he slips the potion into her medicine and waits.
While he is waiting, he sees what he fears is, in actuality, the ghost of Elvira retreat across the room. He pursues it and discovers it is merely Flora, Antonia’s maid, who is spying on him on the advice of Elvira before she died. As they are speaking, Jacintha cries out that Antonia is dying, as it indeed appears.
With her "dying" breath, Antonia confesses how much she admired Ambrosio and desired his friendship, against her mother’s wishes. She leaves everything to her aunt Leonella, and releases her half-uncle Cisternas from all obligations to her, though she waited for him to come rescue her from her dire straits. Thus she appears to die.