The play begins in Naples with Don Juan and the Duchess Isabela who, alone in her palace room, have just enjoyed a night of love together. However, when Isabela wants to light a lamp, she realizes that he is not her lover, the Duke Octavio, and screams for help. Don Juan's uncle, Don Pedro, comes to arrest the offender. But Don Juan cleverly reveals his identity as his nephew and Don Pedro assists him in making his escape just in time. Pedro then claims to the King that the unknown man was Duke Octavio. The King orders Octavio and Isabela to be married at once, with both of them to be held in prison until the wedding.