Act 1
Riccardo is the governor of Boston. In the Senate he has supporters, but there are also some who plot against him. Riccardo's page, Oscar, presents him with the guest list for a ball. Riccardo notices the name of Amelia, his best friend Renato’s wife, whom he secretly loves. He dreams of seeing her again. Renato, who is a political supporter of Riccardo, comes to warn Riccardo about a conspiracy against him. But Riccardo does not want to listen. The Chief Justice appears with a motion to banish Ulrica, a fortune-teller who is spreading dangerous rumours. Oscar defends Ulrica. Riccardo and his entourage decide to visit Ulrica in disguise and form their own impression of her.
Ulrica the fortune-teller is conjuring up the spirits. Riccardo, disguised as a fisherman, mingles with the crowd. The sailor Silvano has his fortune told. With a little help from Riccardo, the prophecy that Silvano will receive gold and a promotion is immediately fulfilled. A servant announces Amelia, who wants to ask for the fortune-teller’s guidance. Ulrica sends everyone away. Only Riccardo remains secretly in the background, and overhears that Amelia wants to end her impossible love for him. Ulrica sends Amelia to a place outside the city to find a healing herb. Riccardo decides to follow Amelia there. When Amelia has left and the people have returned, Riccardo introduces himself as a fisherman, and asks Ulrica to predict the future. Ulrica predicts that he will soon be murdered, by the first friend to shake his hand today. Riccardo refuses to believe this prophecy. When his friend Renato arrives and shakes his hand, he is certain that Ulrica's prophecy is wrong.
Act 2
Amelia is outside Boston, searching for the healing herb that will help her forget her love for Riccardo. Riccardo has followed her. He passionately reaffirms his love for Amelia, and urges her to confess her love too. Unexpectedly, Renato appears, urgently warning Riccardo of an ambush. Amelia veils her face. Riccardo is persuaded to flee, and instructs Renato to take the veiled woman to the city without looking at her. The conspirators Samuel and Tom and their companions arrive, and realise that Riccardo has eluded them. In the melee Amelia's veil is lifted. The conspirators are amused by this discovery, but Renato is deeply shocked by his wife's infidelity and his best friend's betrayal. He asks Samuel and Tom to come to him the next morning.
Act 3
Renato threatens Amelia with death. She begs him to let her see her son once more. Renato diverts his anger away from her, deciding instead to take revenge on Riccardo. When Samuel and Tom arrive, Renato announces that he wants to join their conspiracy. They draw lots to decide who will be permitted to murder Riccardo. Amelia is forced to draw the lot. It falls on Renato. Oscar delivers an invitation to a masked ball at Riccardo's house.
Riccardo decides to renounce Amelia. He wants to send her and Renato to England together. Despite an anonymous warning, Riccardo goes to the masked ball. At the ball, Renato tries to get Oscar to tell him about Riccardo's costume. Oscar refuses at first, but then blurts it out. Amelia and Riccardo recognise each other at the ball. He tells her that she must leave with Renato, and bids her farewell. At that moment, Renato shoots Riccardo. The dying Riccardo swears Amelia’s innocence.