Un Ballo in Maschera follows the story of the assassination of King Gustavo III of Sweden. Gustavo is in love with Amelia, the wife of Anckarström, the king’s secretary and most loyal friend. The lovers meet in secret, even though they know that their love should not continue. Unbeknownst to the king, Count Horn and Count Ribbing are leading a conspiracy against the king to have him murdered. After a trip to see the fortune teller, Ulrica, who tells the king that he will be killed by the hand of a friend, and states that it will be Anckarström. Ignoring this vision, Gustavo continues with his plans for a masked ball, trusting in the loyalty of his friend. However, when Anckarström discovers the affair, he vows to murder the king himself. Just as Gustavo has decided to leave his love for Amelia behind, he is murdered at the ball by Anckarström.