Maria de Vellorno has been spending the interim reprimanding the marquis for allowing the Abate to get the better of him, which he has only been able to do because of the marquis's secret. The marquis’s shame and fury at being compelled into submission convince him that he has only one way out: to murder the imprisoned marchioness and so render the secret powerless by eliminating any evidence of its truth. Thus, the marquis decides to poison his wife.

His plans are disrupted by the revelation of Maria de Vellorno's infidelities when a servant, Baptista, tells the marquis that she meets a young cavalier at the pavilion by the seashore. The marquis hides nearby and catches her. Though he at first decides to forgive her, he is so galled by her denials and tears that he realizes that their relationship is ended forever and disavows a life of dissipated pleasures. He resolves to live entirely for ambition and devotes himself thoroughly to the task of wedding Julia to the duke, through which he will make a powerful alliance. To appeal to the Pope for the return of his daughter, he must first kill the marchioness.