The action unfolds in Genoa and its surroundings in the 14th century.
Twenty-five years pass between the prologue and Act I.
A square in Genoa.
Paolo and Pietro discuss who should be the first Doge of Genoa. Paolo wants it to be Simon Boccanegra, the corsair who has freed Genoa’s sea territory from African pirates.
Boccanegra is a plebeian, and his election should put an end to the power of the patricians. Simon appears. He agrees to Paolo’s entreaties to become the Doge with reluctance: he has no need of authority, but he hopes that in becoming the Doge he can marry his beloved Maria. Her father, the patrician Jacopo Fiesco, does not approve of her marriage to a corsair and keeps his daughter under lock and key.
Paolo and Pietro convince the assembled crowd to elect Simon Boccanegra as the Doge.
Maria has died. Jacopo Fiesco accuses Simon, the father of her child, of his own daughter’s death. Off-stage the chorus laments Maria.
Boccanegra asks Fiesco for a reconciliation. Fiesco is ready to forgive Simon if the latter will surrender Fiesco’s granddaughter to him. But this is impossible: the girl disappeared recently.
Simon enters Fiesco’s palace and sees Maria dead. At the moment the crowd greets the new Doge of Genoa – Simon Boccanegra.
Under Simon Boccanegra’s rule the aristocratic Fiesco and Grimaldi families are exiled. Jacopo Fiesco has taken refuge on the Grimaldi estate under the name of Andrea. Boccanegra’s daughter Maria is also there under the name of Amelia Grimaldi; no-one knows of her true birth.
Amelia awaits her beloved, Gabriele Adorno. Amelia knows that Gabriele and Andrea are preparing a revolt against the Doge and asks Gabriele to abandon this dangerous plan. The Doge arrives at the estate; he plans to ask for Amelia’s hand on behalf of one of his retainers – Paolo. Andrea blesses the marriage of Gabriele and Amelia and tells Gabriele that Amelia Grimaldi is an orphan.
The Doge pardons the Grimaldi family and permits Amelia’s brothers to return to Genoa. In response Amelia tells the Doge about herself and admits that she has a beloved.
Boccanegra recognises her as his daughter Maria. Amelia is delighted – at last she has found her father.
Boccanegra tells Paolo that his marriage to Amelia is impossible. Paolo is unwilling to accept the Doge’s will and plans to abduct Amelia.
The Council Chamber in the Doge’s palace. The Doge and the council discuss affairs of State. The Tatar khan is defeated and the ships of Genoa are free to sail the Black Sea; Francesco Petrarca seeks peace in the name of Venice, but the Genoese councillors call for war.
Cries can be heard from the square – the crowd demands the Doge’s death. Boccanegra decrees that the rebels led by Gabriele Adorno be allowed into the palace.
Amelia has been abducted and Gabriele is convinced that the Doge is responsible. Amelia appears; she has escaped from her captors and confirms that the true criminal is among those present.
Boccanegra asks for Paolo’s help in finding the abductor and makes him curse the evil-doer. Paolo is horrified – he has cursed his own self.
A room in the Doge’s palace.
Paolo is obsessed with a desire for revenge: he adds poison to a decanter lying on the Doge’s table. Pietro brings in Gabriele and Fiesco.
Paolo suggests that Fiesco kills the sleeping Doge, but he refuses to be part of such a dirty piece of work.
Paolo convinces Gabriele that Amelia has become the Doge’s lover, gives him a dagger and slips away.
Amelia appears. She tries to explain that his jealousy is misplaced, but Gabriele does not believe her. The Doge enters and Amelia hides Gabriele on the balcony Amelia tells her father that her beloved in Gabriele Adorno, the leader of the resistance. Boccanegra promises to pardon Gabriele if the latter repents. He drinks the poisoned water and falls asleep.
Gabriele wants to kill the Doge though Amelia tries to prevent him.
The Doge awakes. Realising that Amelia is Boccanegra’s daughter, Gabriele repents and switches sides in support of the Doge.
The revolt again approaches the palace. Gabriele sets out to meet them with an offer of peace.
A room in the Doge’s palace.
The rebellion is crushed. Paolo, who took part in the revolt, is led off to be executed. Before he dies he sees Fiesco and informs him that he organised Amelia’s abduction and poisoned Boccanegra.
A wedding chorus can be heard: Gabriele and Amelia are getting married.
Boccanegra is alone. He senses that soon he will die and with nostalgia recalls the freedom of the sea. Fiesco enters: he wishes to take revenge on Boccanegra and predicts his impending death.
But Simon is pleased to see Fiesco: his daughter has been found and a reconciliation is at last possible. Fiesco forgives his old enemy.
Boccanegra dies in the arms of his daughter, naming Gabriele his successor. Fiesco hails the new Doge.