Un ballo in maschera by Giuseppe Verdi

The story is loosely based on the assassination of King Gustav III of Sweden. The original setting in Sweden was changed to Boston to avoid problems with the censors.

Revolutionaries in Boston are planning to assassinate Riccardo, the city’s governor, but Riccardo dismisses a warning from his secretary, Renato. (As an added plot complication, Riccardo is secretly in love with Renato’s wife, Amelia.)
Accusations of witchcraft are levelled against a fortune teller, Ulrica. Riccardo invites his companions to visit Ulrica to test her powers. He goes ahead, disguised as a fisherman, and hears her prophesying good fortune for a sailor. Riccardo slips money into the sailor’s bag, thus “proving” her uncanny skills. As Ulrica sends the crowd away, Riccardo hides and so witnesses Amelia meeting with the fortune teller. Amelia confesses her love for Riccardo, asking Ulrica for help to vanquish her adulterous passion. She is told to gather a herb found near the gallows. Amelia leaves and Riccardo’s companions arrive.

Riccardo asks Ulrica to read his palm: she foretells his murder by a friend, identifying the assassin as the next person to shake Riccardo’s hand. Renato then arrives and Riccardo greets him with a handshake, claiming the prophecy must be untrue as Renato is his closest friend.
At midnight, Amelia approaches the gallows, seeking the magic herb. Riccardo has come to meet her, and they confess their mutual love. Renato then arrives to warn Riccardo the revolutionary assassins are coming for him. Amelia quickly veils herself and Riccardo leaves, after asking Renato to escort the veiled lady back to the city. When the assassins enter, they taunt Renato for his supposed assignation with a strange woman. He draws his sword and Amelia reveals herself to prevent a fight, provoking the mirth of the conspirators, whom Renato then asks to meet him the next morning.
Renato resolves to kill Amelia for bringing disgrace on their marriage. She pleads to see her son one last time. Seeing a portrait of Riccardo, Renato realises he should rather be taking vengeance on his former friend. The revolutionaries enter. Renato joins their plot, but they argue over who should kill Riccardo. Renato forces Amelia to choose the assassin by drawing lots. She draws Renato’s name! An invitation to a masked ball is brought in and the conspirators see this as their chance.

Amelia warns Riccardo of the plot in an anonymous letter, but Riccardo ignores the threat, not wanting to be seen as a coward. He has decided to send Renato and Amelia back to England, renouncing his love and saving their marriage.
At the ball, Renato persuades Riccardo’s page to reveal his master’s disguise. Meanwhile, Riccardo has recognised Amelia and the two declare their love in a farewell duet. Renato overhears them and stabs Riccardo. In his last moments, Riccardo forgives the conspirators and tells Renato Amelia is innocent of any wrongdoing. He shows them the order for their return to England. The crowd mourns the loss of the magnanimous Riccardo and Renato is tormented by remorse for his actions.