Scene 1: A public audience at Riccardo's palace, attended by his supporters, but also by his enemies who hope for his downfall
Riccardo (Gustavo) reviews the list of guests who will attend an upcoming masked ball. He is elated to see on the list the name of the woman he loves – Amelia, the wife of his friend and advisor, Renato (Count Anckarström). (Aria: La rivedrà nell'estasi / "With rapture I shall look upon her"). When Renato arrives, he tries to warn Riccardo about the growing conspiracy against him (aria: Alla vita che t'arride / "To the life with which you are favoured"), but Riccardo refuses to listen to his words.
Next, Riccardo is presented with a complaint against a fortune-teller named Ulrica (Madame Arvidson), accused of witchcraft. A magistrate calls for her banishment, but Oscar the page defends her (Aria: Volta la terrea / "That tense countenance"). Riccardo resolves to investigate for himself and tells the members of the court to disguise themselves and to meet him at Ulrica's lodging later that day.
Scene 2: At Ulrica's dwelling
Ulrica summons her magical powers: Re dell'abisso, affrettati / "King of the abyss make haste". Disguised as a fisherman, Riccardo arrives before the others. He makes the fortune of a sailor named Silvano come true by spiriting a document of promotion into his pouch, convincing the crowd of the truth of Ulrica's powers. When he realizes that Amelia is coming to see Ulrica, he hides and watches. Alone with Ulrica, Amelia confesses that she is tormented by her love for Riccardo, and asks for a means to bring peace to her heart. Ulrica tells her to gather a certain herb with magical powers; Riccardo resolves to be there when she does so. Amelia leaves.
Now Riccardo presents himself again, along with all of the courtiers, and asks to have his fortune told. (Aria: Di' tu se fedele / "Say whether the sea Awaits me faithfully"). Ulrica reveals that he will be killed by the next man who shakes his hand. He laughingly dismisses her prophecy and offers his hand to the courtiers, who refuse to take it. Renato arrives and shakes Riccardo's hand in greeting. Riccardo's true identity is now revealed and he is acclaimed by the people.
On the outskirts of the town, at the gallows-place. Midnight
Amelia, conquering her fears, has come here alone to pick the herb of which Ulrica told her (Aria: Ma dall'arido stelo divulsa / " If through the arid stalks"). She is surprised by Riccardo, who has come to meet her, and the two finally declare their love for each other.
Unexpectedly, Renato arrives, and Amelia covers her face with her veil before he can recognize her. Renato explains to Riccardo that the conspirators are pursuing him, and his life is in danger. Riccardo leaves, making Renato promise to escort the veiled woman safely back to town, not asking her identity. When the conspirators arrive, they confront Renato; in the struggle, Amelia's veil drops. Renato assumes that Amelia and Riccardo have been involved in an adulterous love affair. He asks the two leaders of the conspiracy, Samuel and Tom, to meet him the next day.
Scene 1: Renato's house
Renato has resolved to kill Amelia for the dishonor she has brought on him. She protests her innocence and begs to see her son one last time. (Aria: Morrò, ma prima in grazia / "I shall die - but one last wish"). Renato relents, and declares that it is Riccardo, not Amelia, who deserves to die (Aria: Eri tu che macchiavi quell'anima / "It was you who stained this soul").
Samuel (Count Ribbing) and Tom (Count Horn) arrive, and Renato asks to join their plot, pledging the life of his son as proof of his sincerity. They agree to draw lots to decide who will kill Riccardo. Amelia is forced to draw the winning name – Renato.
Oscar, the page, arrives with invitations to the masked ball; Samuel, Tom and Renato agree that this is where the assassination will take place.
Scene 2: The ball
Riccardo, torn between love and duty, has resolved to renounce his love for Amelia and send her and Renato back to England (Aria: Ma se m'è forza perderti / "But if I am forced to lose you").
At the ball, Renato tries to learn from Oscar what costume Riccardo is wearing. Oscar at first refuses to tell (Aria: Saper vorreste / "You want to know How he is dressed"), but finally answers: a black cloak and a red ribbon. Riccardo manages to identify Amelia and tells her of the decision he has made. As they say goodbye, Renato stabs Riccardo. The wounded Riccardo discloses that though he loved Amelia, she never broke her marriage vows. He pardons all the conspirators, bidding farewell to his friends and his country as he dies.