Friends and courtiers of Riccardo await him in the throne room of the palace; among them a group of conspirators led by Counts de Horn and Ribbing (Samuel and Tom). As the king enters, his page, Oscar, gives him the guest list for a masked ball. Seeing the name of Amelia - wife of his first minister, Renato - he muses on his secret passion for her ("La rivedra nell'estasi"). As the others leave, the page admits Renato himself, who says he knows the cause of the king's disturbed look: a conspiracy against the crown. But Gustav ignores his friend's warning.
A magistrate arrives with a decree banishing the fortune teller Ulrica, who has been accused of witchcraft. When Gustav asks Oscar's opinion, the youth describes her skill at stargazing and urges him to absolve her of any crime ("Volta la terrea"). Deciding to see for himself, and overruling the objections of Renato, the king light-heartedly bids the court join him in an incognito visit to the soothsayer.
As Ulrica mutters incantations before a group of women ("Re dell'abisso"), Gustav discreetly enters disguised as a fisherman. The fortune teller begins her prophecies by telling the sailor Silvano that he will soon prosper. Gustav surreptitiously slips money and a promotion into the satchel of the seaman, who discovers it and marvels at the fortune teller's powers. The king stays in hiding when Ulrica sends her visitors away to grant an audience to Amelia, who comes seeking release from her love for Gustav. Ulrica tells her she must gather at night a magic herb that grows by the gallows; Amelia hurries away as Gustav, having overheard the conversation, resolves to follow her. A moment later Oscar and members of the court enter, and Gustav, still disguised as a fisherman, mockingly asks Ulrica to read his palm ("Di' tu se fedele"). When she says he will die by the hand of a friend, the king laughs (Quintet: "E scherzo od e follia"). Still incredulous, Gustav asks her to identify the assassin, to which she replies that the next hand he shakes is the one that will kill him. No one will shake "the fisherman's" hand, but upon seeing Renato arrive, he hurries to clasp his hand and says that the oracle is now disproved since Renato is his most loyal friend. Gustav is recognized, and is hailed by the crowd above the muttered discontent of the conspirators.
Amelia arrives by the gallows and desperately prays that the herb she seeks will release her from her passion for the king ("Ma dall'arido stelo divulsa"). As a distant bell tolls midnight, she is terrified by an apparition and prays to heaven for mercy. Gustav arrives, and unable to resist his ardent words, Amelia confesses she loves him (Duet: "Non sai tu che se l'anima mia") but quickly veils her face when her husband rushes in to warn the king to flee approaching assassins.
Gustav, fearing that Renato may discover Amelia's identity, leaves only after the Captain promises to escort her back to the city without lifting her veil. Finding Renato instead of their intended victim, the conspirators curse their luck. The husband draws his sword when they make insolent remarks about his veiled companion; to save her husband's life, Amelia raises her veil. While the conspirators laugh at this irony, Renato asks their two leaders to come to his house the next morning and Amelia laments her disgrace.
Dragging Amelia into their home, Renato tells her that he intends to kill her; Amelia asks to see her young son before she dies ("Morro, ma prima in grazia"). Granting her wish, Renato turns to a portrait of Gustav and exclaims that it is not on Amelia that he should seek vengeance, but on the king ("Eri tu"). He is interrupted by de Horn and Ribbing; now united in purpose, they cannot agree who should have the privilege of assassinating the king. Amelia returns just as the men prepare to draw lots. Forcing his wife to choose the fatal slip of paper from a vase, Renato rejoices when she draws his name. A moment later, Oscar brings an invitation to a masked ball at the opera house. While the men hail this chance to execute their plan, Amelia plans to warn Gustav (Quintet: "Di che fulgor").
Alone in his apartment, Gustav resolves to renounce his love, and to send Amelia and Renato to Finland. ("Ma se m'e forza perdeti"). Oscar delivers a letter to the king from an unknown lady warning him of the murder plot. Not wanting his absence to be taken as a sign of cowardice, Gustav leaves for the masquerade. In the Royal Opera House ballroom, festivities are in progress. The three conspirators wander through the crowd trying to learn the disguise of the king. Renato, taking Oscar aside, tries to persuade the youth to reveal the king's identity and is successful only after the boy's playful evasions ("Saper vorreste"). Recognizing Amelia, Gustav speaks with her (Duet: "T'amo, si, t'amo"); despite her repeated warning, he refuses to leave. Just as the lovers bid a final farewell, Renato, overhearing the last part of their conversation, plunges his dagger into the king. The dying Gustav forgives Renato, and admits he loved Amelia but assures the remorseful captain of his wife's innocence. The crowd bewails the loss of such a generous-hearted king.
A Masked Ball