Before the action takes place, a battle has been fought between Oberto, Count of San Boniface, and the Salinguerra, led by Ezzelino da Romana. Oberto has lost and has retreated to Mantua. Meanwhile, his daughter Leonora has been seduced and abandoned by Riccardo, Count of Salinguerra, and Riccardo is about to marry Cuniza, Ezzelino's sister. Leonora makes her way to Bassano on Riccardo's wedding day, intent on confronting him.
Scene 1: The countryside near Bassano
Riccardo is welcomed by a chorus as he is about to enter Ezzelino's palace. He sings of his joy at being close to Cuniza (Son fra voi! Già sorto è il giorno...Già parmi udire il fremito - "Here I am amongst you! The day hasted by my desire has now arrived"). They enter the castle. Leonora arrives swearing to avenge Riccardo's desertion and she sings of the love which she had and a hope of recovering those innocent days (Sotto il paterno tetto...Oh potessi nel mio core- "Beneath my father's roof an angel appeared to me"). She leaves to go towards the village.
Scene 2: Near Bassano castle
Meanwhile her father, Oberto, arrives pleased to be back in his home country but unsure of Leonora's whereabouts. When Leonora returns, each is aware of the other's presence and father and daughter are reunited. They express amazement at having found each other again. But, Oberto's initial anger at Leonora's actions quickly turn to fatherly affection as the pair makes plans to disrupt the wedding.
Scene 3: A room in Ezzelino's palace
The chorus sings a welcome to the happy bride but, alone with Riccardo, Cuniza expresses some forebodings in spite of expressing her love for him. (Questa gioia che il petto m'innonda - "This joy that overwhelms my breast is mingled with a mysterious fear").
After the couple leaves, Leonora enters and is questioned by Imelda. Leonora tells her that her father, Oberto, is also in the palace and, When he enters, she tells Cuniza about her betrayal by Riccardo. Cuniza agrees to help them.
She then hides Oberto in a nearby room and invites Riccardo and his guests to join her. Open entering, Cuniza reveals Leonora's presence and accuses her lover of infidelity. Riccardo's accusations against Leonora prompt her father to enter and challenge Riccardo to a duel.
Scene 1: The princess's private apartments
Cuniza and Imelda are alone and the servant announces that Riccardo wishes to speak to her mistress. Cuniza laments the love she had, but following her aria, Oh, chi torna l'ardente pensiero - "Oh, who can turn my fevered thoughts", she instructs Imelda to tell Riccardo that he should return to Leonora, concluding in Più che i vezzi e lo splendore - "More persuasive to my conscience" that she has made the right decision.
Scene 2: A remote place near the castle gardens
The courtiers gather and express their sympathy for Leonora's plight. As they leave, Oberto enters, waiting for his rival to appear. He proclaims that he will seek vengeance: (Aria: L'orror del tradimento - "The horror of his betrayal"). The courtiers return to tell him that Cuniza has interceded on his behalf and that he has nothing to fear from Riccardo, but Oberto's thoughts still focus on vengeance. Riccardo finally arrives and they begin to fight, but are soon stopped Cuniza who arrives with Leonora. The former insists that Riccardo admit his infidelity and agrees to marry Leonora. Still determined to fight his rival, Oberto extracts an agreement from Riccardo that they will soon meet in the woods. Oberto leaves the group to go into the woods; all leave. Offstage, the sound of a duel in progress can be heard and it is followed by Riccardo's arrival. He realizes that he has killed Oberto (Aria: Ciel che feci? - "Heavens, what have I done!") and he is filled with remorse. Then Imedla and Cuniza appear explaining that Leonora is prostrate over the dead body of her father. Soon a letter arrives from Riccardo explaining that he has gone into exile leaving everything he possesses to Leonora. In her grief, she announces that she will become a hermit.