Paolo, the leader of the Plebian party, persuades Pietro to support the nomination of Simon Boccanegra for doge of Genoa. Boccanegra arrives and agrees to stand, thinking that Fiesco would then allow him to wed his daughter, Maria, who is being held prisoner in her father's gloomy palace because she bore Boccanegra an illegitimate child. Pietro rallies support for Boccanegra. Fiesco enters, stricken with grief over his daughter's death (Il lacerato spirito - "The tortured soul of a sad father"), but he does not reveal this to Boccanegra who accosts Fiesco and begs his forgiveness. Fiesco promises clemency only if Boccanegra lets Fiesco have his granddaughter. Boccanegra explains he cannot because the child has vanished. As the people hail Simon as the new Doge, he finds the body of his beloved.
Scene 1: Twenty-five years later
The Doge has exiled many of his political opponents and confiscated their property. In the Grimaldi castle, Fiesco, to avoid discovery, is using the name Andrea Grimaldi, plotting with Boccanegra's enemies to overthrow him. Unknowingly, years earlier, the Grimaldis had adopted Boccanegra's child (and Fiesco's granddaughter) after discovering the orphan in a convent. They called her Amelia, hoping that she would be the heir to their family's fortune, their sons having been exiled. Amelia awaits her lover, Gabriele Adorno (Aria:Come in quest'ora bruna - "How in the morning light / The sea and stars shine brightly"). He arrives, and she warns him of the dangers of political conspiracy. Word arrives that the Doge is coming. Amelia, fearing that a forced marriage to Paolo is to be arranged, urges Adorno to ask her father for permission to marry. Fiesco agrees and reveals that Amelia is actually a penniless foundling. When Adorno says that he does not care, Fiesco blesses the marriage. Boccanegra enters. He pardons Amelia's exiled brothers, but she refuses to marry Paolo. When she tells Boccanegra that she was adopted, the two compare pictures in their lockets and realizes that she is his long-lost daughter. Finally reunited, they are overcome with joy. When Paolo enters, Boccanegra denies permission for the arranged marriage. Furious, Paolo decides to kidnap Amelia.
Scene 2: The senate is in session
The Doge is interrupted by the sounds of a mob demanding Boccanegra's head. He orders the doors opened, and the crowd bursts in, chasing Adorno. Adorno confesses to killing Lorenzino for the attempted kidnapping of Amelia, ordered by an unknown high ranking official. Adorno guesses it must be Boccanegra and is about to attack him when Amelia rushes in and stops the fight (Aria: Nell'ora soave - "At that sweet hour which invites ecstacy / I was walking alone by the sea"). Boccanegra has Adorno arrested for the night (Aria: Plebe! Patrizi! Popolo! - "Plebians! Patricians! Inheritors / Of a fierce history"). Discerning that Paolo is the actual man responsible, he makes everyone, including Paolo, utter a curse on the real kidnapper.
Paolo and Fiesco discuss plans to murder Boccanegra, but Fiesco refuses. Paolo next tells Adorno that Amelia is the Doge's mistress, hoping Adorno will murder Boccanegra. Just before Amelia enters, Adorno's anger and jealousy prompts an angry outburst (Aria: Sento avvampar nell'anima - "I feel a furious jealousy / Setting my soul on fire"). Amelia enters, and Adorno accuses her of infidelity. She claims only to love Adorno, but does not explain that Boccanegra is her father for Adorno's family was killed by the Doge. Adorno hides as Boccanegra enters. Amelia vows to Boccanegra that she would die for Adorno. Boccanegra agrees to pardon him. He drinks from a poisoned glass of wine, which Paolo has previously placed on the table, and falls asleep. Adorno tries to kill him, but Amelia stops him. Boccanegra wakes and reveals that Amelia is his daughter. Adorno begs for forgiveness (Aria: Perdon, Amelia... Indomito - "Forgive me, Amelia... A wild, / Jealous love was mine") and he promises to fight for the Doge.
Paolo is condemned to death for leading the uprising against the Doge. Fiesco is released from prison. Paolo tells Fiesco that he has poisoned Boccanegra. Fiesco confronts Boccanegra, who is now dying. Boccanegra recognizes his old enemy, but is happy to tell him that Amelia is his granddaughter. Fiesco feels great remorse and tells Boccanegra about the poison. Adorno and Amelia, newly married, find her father and grandfather have reconciled. Boccanegra asks that Adorno be named his successor, and after the Doge dies, Fiesco proclaims it so.