Background: The opera is set in 14th-century Genoa, in an atmosphere of intense political and class rivalries. The ruling patricians are led by Jacopo Piesco and include several powerful families, such as the Grimaldis and the Dorias. The plebians are led by Pietro, Paolo, and Simon Boccanegra, a sea captain. Boccanegra has had an affair with Piesco’s daughter Maria, resulting in the birth of a girl, also named Maria. Genoa is about to elect its first Doge and the plebians are determined to seize power by electing one of their own.

Prologue: A square in Genoa at nighttime. Paolo and Pietro suggest to Boccanegra that he be nominated as the new Doge. At first he refuses, wanting instead to find his beloved Maria, but finally agrees. Piesco appears, grieving over the sudden death of his daughter. Piesco confronts Boccanegra, demanding that the child Maria be returned. Boccanegra explains that she has vanished. Piesco proclaims that there will never be peace between them, while Boccanegra is proclaimed the new Doge.

25 years elapse before Act I. Piesco has gone into hiding with the Grimaldi family and now goes by the name Andrea. They have raised an orphan girl, named Amelia, and Andrea has adopted her. She is, in reality, the grown-up Maria, his granddaughter.  Amelia is engaged to Gabriele Adorno, a young Genoese patrician.

Scene 1. The garden of the Grimaldi family palace outside Genoa. Amelia warns Gabriele of the dangers in plotting against the Doge. Boccanegra arrives to secure Amelia’s hand in marriage to Paolo. Amelia tells Boccanegra that she is in love with someone else and also shows him a locket containing a portrait of her mother. When Boccanegra produces an identical locket, they realize they are father and daughter.

Scene 2. The Council Chamber inside the Doge’s Palace. The Council meeting is interrupted by a mob which enters, dragging in Gabriele and Piesco, whom they accuse of trying to abduct Amelia. Before Amelia can name her abductor, the two forces begin to accuse one another. Only Boccanegra’s intervention prevents bloodshed. He implores them to work together to restore peace to the city and orders everyone to curse the would-be abductor.

The Doge’s living quarters inside the Palace. Intent on revenge, Paolo pours poison into the Doge’s water goblet. Amelia tells her father that she loves Gabriele, but Boccanegra names him as one of the rebels about to overthrow the government. Boccanegra sips from his water goblet and falls asleep. Gabriele is about to stab Boccanegra when Amelia returns and throws herself between them. Boccanegra awakens and tells Gabriele the truth about Amelia’s past. The conspirators are heard outside. Boccanegra instructs Gabriele to join his colleagues but he refuses, pledging himself to the Doge. Boccanegra promises him Maria’s hand in marriage.

Inside the Doge’s Palace. Boccanegra’s forces have quelled the rebellion. Paolo, who joined the conspirators, is executed after confessing that he poisoned Boccanegra. Still intent on vengeance, Piesco confronts Boccanegra, who explains that Maria is his long-lost daughter and therefore Fiesco’s granddaughter. Just before he dies, Boccanegra asks that Gabriele be appointed Doge, which will bring together the plebian and patrician factions.