Genoa, 14th century. Paolo and Pietro, leaders of the plebeians, are conspiring to overthrow the aristocracy. They name the popular former pirate Simon Boccanegra as their candidate for the office of doge, the chief magistrate of the republic. Boccanegra accepts, hoping that his new position will enable him to marry Maria, the daughter of the patrician Fiesco, who keeps her prisoner because she bore Boccanegra an illegitimate child. Fiesco appears alone, mourning Maria’s sudden death. Unaware she has died, Boccanegra tries to make peace with the patrician, but Fiesco demands that he first be given his granddaughter. Boccanegra explains that the little girl has mysteriously disappeared. The two men part and Boccanegra enters the Fiesco palace in search of Maria. Just as he discovers her body, the crowd proclaims him doge.

Twenty-five years have passed. Boccanegra has exiled many of his political opponents and Fiesco lives outside Genoa under the assumed name of Andrea Grimaldi. He is the guardian of a certain Amelia Grimaldi. Abandoned as an orphan, she has been brought up in place of the real Grimaldi daughter, who died at a young age, in order to provide the family with an heiress. Amelia is in reality Maria Boccanegra, the doge’s daughter and Fiesco’s granddaughter, but neither man knows her true identity. Amelia’s lover is the patrician Gabriele Adorno. Together with Fiesco, whom he knows only under his assumed name, Gabriele has been plotting against Boccanegra.

In the garden of the Grimaldi palace, Amelia waits for Gabriele. She warns him against the dangers of his political activities and tells him that the doge wants her to marry his courtier Paolo. Gabriele hopes to marry Amelia himself and is undeterred by her revelation that she is not a Grimaldi but an orphan of unknown background. He leaves, determined to overthrow the doge. Boccanegra arrives to tell Amelia he has pardoned her presumed brothers, the Grimaldi. Impressed by his generosity, she admits her love for Gabriele and talks about her lonely past. From the matching portraits they have of Amelia’s mother, Boccanegra realizes that Amelia is his long-lost daughter. They embrace. The girl leaves and Boccanegra tells Paolo to abandon any hope of marrying her. Paolo plots with Pietro to kidnap Amelia. 

Boccanegra urges the city council to preserve peace with Venice. Gabriele runs in, chased by a mob: he has killed a man who was attempting to abduct Amelia. He accuses Boccanegra of plotting the abduction and tries to stab him. Amelia intervenes. She describes her abduction and escape, but refuses to reveal who was responsible. A new argument erupts and Boccanegra again urges peace. He curses Amelia’s kidnappers and commands Paolo, whom he suspects to be the culprit, to repeat his words. The terrified Paolo is forced to obey, even though he is cursing himself.

Paolo reflects on the curse and pours poison into Boccanegra’s drinking water. Fiesco and Gabriele are led in, and Paolo tries to convince the old man to assassinate the doge, but in spite of his hatred for Boccanegra, Fiesco refuses to be involved in the plot. When Paolo makes insinuations about Boccanegra’s relationship with Amelia, Gabriele, left alone, breaks into a fit of jealousy. Amelia enters, but before she can explain, Boccanegra appears. Gabriele hides while Amelia asks her father to pardon her lover. Boccanegra agrees. Alone, he drinks the poisoned water and falls asleep. Gabriele, who has heard nothing of the preceding conversation, returns and is about to stab Boccanegra when Amelia rushes in. The doge reveals that he is Amelia’s father and forgives the repentant Gabriele. As a rebellious mob is heard from outside, Gabriele vows to fight at Boccanegra’s side.