Act I
Scene 1: An audience chamber in the royal palace, Stockholm, 1792

King Gustavus is meeting with his courtiers, who are planning a masked ball for the following night. The king’s young page, Oscar, gives him the guest list, and when he sees the name of Amelia (Countess Anckarstroem), he stops to muse about his secret love for her. When the courtiers leave, Count Anckarstroem enters and warns the king that there is a rebellion brewing among the aristocracy, who resent the progressive king for diminishing their ancient privileges. Gustavus laughingly ignores the warning. A judge enters who wants to exile a fortuneteller named Ulrica, who has been accused of witchcraft.  Oscar, the page, defends the woman, saying she is merely a skilled stargazer. Gustavus becomes curious and decides to visit Ulrica’s hut in disguise, over the objections of Count Anckarstroem. 

Scene 2: The den of the fortune teller

Ulrica is muttering incantations, and she tells a sailor, Christian, that he will soon prosper. Gustavus hides as a servant announces Amelia, who is seeking a remedy for her love for someone other than her husband. Ulrica tells her to seek at midnight a magic herb that grows only by a gallows. When Amelia leaves, Gustavus, now joined by Oscar and other courtiers, asks Ulrica to tell his fortune. Ulrica reluctantly reads his palm and says he will be murdered by the next person to shake his hand. No one present will accommodate Gustavus when he extends his hand until Count Anckarstroem arrives and, unaware of what has been foretold and happy to find the king unharmed, shakes his hand. Everyone laughs off the prediction because Anckarstroem is the king’s most loyal confidant.   

Act II
Scene 1: A lonely field near a gallows

Amelia arrives to search for the magic herb. As a bell tolls midnight, she is frightened by the approach of a shadowy figure. She realizes it is Gustavus and tries to resist his ardent entreaties. Amelia at last confesses her love to him but says she will not act upon it and begs him to leave. Count Anckarstroem suddenly appears to warn the king that assassins are nearby. Amelia quickly veils her face. Anckarstroem convinces the king to flee by a different route and offers to escort the unknown lady back to town. The conspirators arrive and threaten Anckarstroem. Rushing to his aid, Amelia lets her veil slip. Anckarstroem is bitterly humiliated when he discovers her identity. 

Scene 2: The study in the Anckarstroem home              

In a jealous rage, Anckarstroem drags Amelia into their house and threatens to kill  her. She pleads for the chance to say farewell to their young son. He allows her to leave the room to do so and then turns to vent his anger on a portrait of the king, vowing to join the assassins in the conspiracy. When Amelia returns to the room, she finds her husband with the conspirators. She is forced to draw lots to determine who will kill the king and draws Anckarstroem’s name. He rejoices at the chance for revenge. Oscar arrives with an invitation to the masked ball, and the conspirators decide to execute their plan at the event. Amelia is determined to warn the king. 

Act III
Scene 1: The king’s apartment in the palace

Alone, Gustavus signs a document sending the Anckarstroems abroad—trying to overcome his love for Amelia. Oscar delivers a letter to the king from an unknown lady, warning of the plot to kill him. Gustavus, not wanting to appear cowardly, decides to go to the ball.  

Scene 2: A ballroom in the palace

At the masquerade ball, Gustavus looks for Amelia while Anckarstroem tricks Oscar into revealing the king’s disguise. Gustavus recognizes Amelia and, in spite of her repeated warnings, he refuses to leave the ball. As the king and Amelia tenderly say a final goodbye, Anckarstroem, overhearing the last of their conversation, shoots Gustavus. The king, surrounded by his court, assures Anckarstroem that Amelia is innocent of any infidelity whatsoever. Before dying, Gustavus forgives Anckarstroem and bids his courtiers to spare the count’s life.