The governor of the city, Riccardo, is secretly in love with Amelia, the wife of his best friend and secretary, Renato. While investigating the suspicious political activities of the fortune-teller, Ulrica, Riccardo overhears Amelia privately ask for a remedy to cure a hopeless love. After Amelia leaves, a disguised Riccardo asks Ulrica to read his palm, but scoffs at her warning that he will be killed by a friend. Asked to identify the killer, she tells him it will be the next person to grasp his hand. Renato arrives and shakes Riccardo's hand in greeting, an action Riccardo then touts as proof that Ulrica's warning is fiction since Renato is his most loyal friend.

Following Ulrica's instructions, Amelia visits the gallows at midnight seeking a magic herb and is surprised there by Riccardo. In a moment of passion they declare their love, but are interrupted by the arrival of Renato, who has followed Riccardo to warn him of a conspiracy he has uncovered. Renato is told to take the veiled woman back without asking her name, but as they return they are accosted by the conspirators and Amelia's identity is revealed.

While awaiting the masked ball that evening, Riccardo reluctantly decides he cannot compromise Amelia's life and resolves to assign Renato and his family to a distant post. Renato, believing himself betrayed by both his wife and friend, joins lots with the conspirators. That evening, just as Riccardo informs Amelia of the new assignment and bids her goodbye, Renato enters and stabs him. Mortally wounded, Riccardo forgives Renato and admits to loving Amelia, but assures Renato of her innocence before he dies.