The Governor of Boston, Riccardo Earl of Warwick, receives his courtiers. He looks down the list of those invited to the ball and sees the name of Amelia, with whom he has long been secretly in love. The trouble is that Amelia is married – to his secretary Renato, who is also his best friend. Suddenly Renato himself appears to warn the Earl of a plot and to reveal the names of the traitors. Despite Renato’s insistence, Riccardo refuses to listen to anything about conspirators. At that moment Oscar, Riccardo’s faithful page, announces the arrival of the chief judge, who wishes to obtain the Governor’s signature on an order to banish the sorceress Ulrica from the city. Seizing the opportunity, the page intervenes on the sorceress’ behalf, going into raptures about her gift of prophecy. Riccardo is amused by all this, and suggests that everybody disguise themselves and visit the fortune-teller.
Ulrica’s hut. Among those awaiting prophecies is Earl Riccardo, dressed as a fisherman. Amelia also arrives to seek the sorceress’ help in getting rid of her secret and forbidden love – her love for Earl Riccardo. Ulrica orders Amelia to go to the place where death sentences are carried out and gather a magic herb that will make her forget. After Amelia has left, Riccardo asks Ulrica to read his palm. The sorceress looks at his palm, then suddenly stops, looks sad, and refuses to tell him what she sees. Finally, at the Earl’s insistence, Ulrica reveals that he will soon die – not in battle, but at the hand of a friend – the one who is the first to shake his hand.
Everyone is horror-struck, but Riccardo refuses to believe the prophecy. He holds out his hand, but nobody wants to shake it. At that moment Renato enters, greets the Earl and shakes his hand to the delight of his friends and the disappointment of the conspirators, because no one believes that the devoted Renato could possibly murder the Earl. Riccardo nevertheless pays Ulrica for her fortune-telling. The choir sings the Earl’s praises.
Midnight. In wasteland outside the city Amelia wanders in search of a magic herb and unexpectedly meets Riccardo, who has followed her. In tears, Amelia begs him to leave. She no longer has the strength to suppress her feelings for him. However, just at the moment when they are declaring their love for one another, Renato appears. Terrified, Amelia covers her face with a veil. Renato warns Riccardo that Samuel and Tom want to kill him. Riccardo has to go away, leaving his beloved with her husband, who has not recognised her. But before he departs, Riccardo makes Renato swear that he will escort the lady back to the city, not attempting to find out who she is. Renato swears he will, and intends to hide her in a safe place. However, the conspirators Tom and Samuel prevent Renato from carrying out his intention. They have sought out Renato, knowing that their plans have been exposed, and now want to learn the identity of his mysterious companion. Renato tries to protect the lady, but in vain. When the conspirators start threatening Renato with death, Amelia lifts her veil herself. In astonishment Tom and Samuel subject the Governor’s secretary to ridicule. Renato, who is filled with hate and desire for revenge, invites the conspirators to visit him the next day and returns to the city with Amelia.
Renato’s house. The deceived husband is ready to kill his wife there and then, but, heeding Amelia’s entreaties, he allows her to see her son for the last time. Left in solitude, Renato comes to his senses. His gaze rests on a portrait of Riccardo Earl of Warwick, and he decides that it would be better to kill him.
Tom and Samuel enter. Renato declares his readiness to join their plot. In order to convince the conspirators, he is prepared to swear on his son’s life. Amelia is forced to draw lots to establish who is to kill Earl Riccardo. The lot falls to Renato. Oscar arrives with an invitation to a masked ball to be held that evening. Renato decides that this will be the appropriate moment for retribution – the mask will help him carry out the murder. Meanwhile, Earl Riccardo is taking the decision to part with Amelia forever. He wishes to send her and her husband to England. A sense of foreboding is replaced by the desire to see her again. Oscar the page passes Riccardo an anonymous letter which states that somebody will try to kill him at the ball. But Riccardo pays no attention to the warning: all he wants is to see Amelia once more. A hall in the Governor’s palace. The masked ball is in full swing. Renato, learning from Oscar what costume the Earl is wearing, seeks him out among the guests and finds him at the very moment when he is declaring his love for Amelia for the last time and saying goodbye to her. Renato stabs his friend with a dagger and attempts to hide, but the Governor’s guests catch the murderer and tear off his mask.
Dying, Riccardo begs them to release his friend and tells him of the innocent nature of his love for Amelia, and that he had parted with her forever.