On the night of the Carnival, Simone searches his house for his wife Violanta in vain. Ever since her sister Nerina committed suicide after being seduced by Alfonso, the Prince of Naples, Violanta has vowed to avenge her.
Simone is about to leave for the Carnival with the painter Giovanni Bracca when Violanta appears. After dismissing Bracca, she admits to Simone that she has anonymously arranged a meeting with Alfonso in their house, where she wants her husband to kill him. Initially horrified at this plan, Simone finally caves in. At Violanta’s threat that she could succumb to Alfonso’s wooing, for hate and love are feelings of the same nature, Simone swears that Alfonso will not leave the house alive. Once Alfonso will be unarmed, Violanta will sing the same Carnival song with which she had first lured him in: at that signal, Simone will enter to murder him.
Alfonso arrives. During their meeting, Violanta reveals her true identity and her intention to avenge Nerina. However, when Alfonso relates the story of his unfulfilling life and admits his longing for death, Violanta realises that she is actually in love with him – as he is with her. She hesitates to give Simone the signal to come out and kill him. Simone becomes impatient and enters, finding the lovers in embrace. As Simone is about to strike Alfonso, Violanta interposes herself, receiving the blow.
Outside, the colourful carnival hustle can be heard. Giovanni rushes into the house with a group of masks. Silence sets in as Violanta dies in Simone’s arms.