Act I
In the temple of Solomon, Jews lament the fate of the kingdom of Israel, which is under threat from the king of Assyria Nebuchadnezzar (Nabucco). High Priest Zaccaria arrives with news to partially revive spirits; the Jews are holding Fenena, Nabucco’s daughter, hostage. Perhaps with his mediation, the Assyrians will withdraw. The arrival of Ismaele, however, returns the anguish as he bears bad news: Nabucco’s arrival is imminent. Zaccaria entrusts Fenena to Ismaele, promising that the God of Israel will prevail over that of Baal. Ismaele is delighted for he has been in love with Fenena ever since she has saved him twice: once from Nabucco’s prison and then from the mad love of her elder sister, Abigaille. Of course, Fenena is now a slave, and she reluctantly reminds Ismaele of this. Undeterred, he swears to her freedom, but Abigaille appears before them along with Assyrian soldiers disguised as Jews. Upon seeing them, Abigaille accuses Ismaele of betraying his people and declares her love for him. A love that has now turned to hate. Meanwhile, the troops commanded by Nabucco are approaching. Abigaille precedes him and helps the Assyrians break into the temple. In the ensuing panic, Zaccaria reminds Nabucco that he has Fenena in his hands and threatens to kill her if he dares to desecrate the temple. Surprisingly, it is Ismaele who stops Zaccaria. Fenena runs into her father’s arms and Nabucco promises revenge. Zaccaria and the Jews curse Ismaele.
Act II
Abigaille has stolen a document from her father which shows she is not the king’s daughter but the child of slaves. She swears vengeance on Nabucco and his appointed heiress, Fenena. The High Priest of Baal, who sides with Abigaille, comes to say that Fenena has freed the Hebrew prisoners. As a result of her treason, the religious authorities have decided to offer Abigaille the throne instead, telling the people that their king has fallen in battle. She relishes the idea of the slave who becomes sovereign. Zaccaria, meanwhile, announces that Fenena, thanks to her love for Ismaele, has been converted to their faith. The aged palace adviser Abdallo rushes in to tell Fenena about the reports of the king’s death and to warn that her life is in danger. Before she can escape, the High Priest of Baal, followed by Abigaille and the Assyrian populace, proclaims Abigaille ruler and pronounces a death sentence on the Hebrews. Unexpectedly, Nabucco arrives: he takes the crown and places it on his own head. Everyone quakes in dread before the irate monarch, who announces he is not only a king but god, having overthrown both Baal and Jehovah. As he tries to force Zaccaria and Fenena to prostrate themselves, lightning strikes him. When he recovers, he seems upset and deprived of his senses and calls out to Fenena for help. Amidst the confusion, Abigaille retrieves the crown and disappears.
Act III
In the Hanging Gardens of Babylon, the High Priest of Baal and the populace hail Abigaille as ruler. The dishevelled Nabucco wanders in, hoping to sit once more on his throne. Abigaille dismisses the others and explains to Nabucco that she is serving as regent since he is not well enough to rule. Taking advantage of his insanity, she tricks him into signing the warrant condemning the Jews to death. Then Nabucco is struck with the realisation that his own daughter Fenena will also die. When Nabucco tries to find in his garments the document proving that Abigaille is an impostor, she confronts him with it and tears it to bits.
Nabucco is imprisoned. Desperate, he asks that at least Fenena be returned to him. On the banks of the Euphrates, the Hebrews are resting from forced labour. Their thoughts ascend ‘on golden wings’ to their lost homeland. Zaccaria predicts they will overcome captivity and obliterate Babylon with the Lord’s help.
Act IV
In his palace, Nabucco sees Fenena being dragged to her execution. Desperate, he kneels to pray to the God of the Hebrews for forgiveness, pledging to convert himself and his people. His reason returns. Crying for a sword, he rallies Abdallo and a group of faithful soldiers to regain the throne. In the Hanging Gardens, executioners stand ready to do away with Zaccaria and his flock. The old man hails Fenena as a martyr, and she asks the Lord to receive her into heaven. However, Nabucco arrives and orders the statue of Baal destroyed. As if by supernatural powers, it falls of its own accord and breaks. Abigaille takes poison and confesses her crimes, urging that Ismaele and Fenena be reunited. Dying, she prays to the God of Israel to pardon her.
Nabucco tells the Jews to return to their native land and rebuild their temple, declaring that he himself now serves Jehovah. The crowd acknowledges a miracle and renders praises to God.
