In a piazza in fourteenth-century Genoa, Paolo and Pietro, leaders of the popular (plebeian) party, conspire to gain power over the aristocracy (patricians). 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 position will enable him to marry Maria, who has been imprisoned by her father, the patrician Fiesco, because she bore Boccanegra an illegitimate child. The plebeians pledge their support to Boccanegra. When they leave, Fiesco appears, mourning Maria’s death. Boccanegra, unaware she has died, returns and tries to make peace with the patrician. Fiesco demands that he first be given his grandchild, but Boccanegra explains that she has disappeared. Fiesco leaves and Boccanegra enters the palace, discovering Maria’s body. As he staggers outside, the crowd proclaims him doge. Twenty-five years elapse between the Prologue and Act I.
Act I. Amelia waits for her lover Gabriele in the garden of the Grimaldi Palace. He arrives, and she warns him against the dangers of plotting against the doge. Gabriele learns from Amelia that the doge wants her to marry his courtier Paolo. Gabriele resolves to obtain the blessing of her guardian, Andrea. The young man is not dissuaded by Andrea’s revelation that Amelia is not a true Grimaldi but an orphan of unknown background. Determined to overthrow the doge, the men leave just as a fanfare announces Boccanegra, who tells Amelia he has pardoned her foster brothers. She admits her love for Gabriele and describes her lonely past. Boccanegra produces a locket with a likeness of the dead Maria and finds that Amelia has an identical locket. Boccanegra realizes that Amelia is his long-lost daughter Maria. When the doge tells Paolo to forget his dream of marrying Amelia, Paolo plots with Pietro to kidnap her. Gabriele, chased indoors by the mob, has killed a man who was attempting to abduct Amelia. He accuses Boccanegra of plotting the abduction. As Gabriele tries to stab the doge, Amelia intervenes, pleading for the life of Gabriele, who suspects her of being Boccanegra’s mistress. Amelia describes her abduction to the Council, hinting at Paolo’s complicity, and Boccanegra urges peace for the warring factions. He commands Paolo to curse the man behind the kidnapping, and the terrified Paolo is forced to obey.
Act II. In the doge’s chambers, Paolo sends Pietro to free Gabriele and Andrea from prison. Remembering the curse, he pours poison into Boccanegra’s water pitcher. When the two prisoners enter, Paolo tries to convince the old man to assassinate the doge, inciting Gabriele with insinuations about the doge’s relationship with Amelia. Gabriele raves with jealousy until Amelia enters; before she can explain, Boccanegra is heard approaching. Gabriele hides while Amelia asks the doge to pardon her lover. Boccanegra agrees on the condition that Gabriele leave the conspirators. Gabriele, who has heard nothing, enters and is about to stab Boccanegra when Amelia rushes in and stops him. The doge awakens and reveals to Gabriele that he is Amelia’s father. A rebellious mob gathers outside, and Gabriele says he will pacify them or die in Boccanegra’s defense. The doge offers him Amelia’s hand in marriage as his reward.
Act III. Genoa is celebrating Boccanegra’s victory. Andrea, set free, encounters Paolo on his way to execution. Paolo admits that he poisoned the doge. Boccanegra staggers in, mortally ill. Andrea reveals his identity as Fiesco, and learns from the doge that Amelia is his granddaughter. The old man weeps at learning the truth too late. Dying, the doge blesses the young couple, naming Gabriele his successor. Fiesco sadly announces Boccanegra’s death to the people.