In the Temple of Jerusalem.
Nebuchadnezzar, the mighty king of Babylon, is at the gates of the city. The Levites and the people are on their knees, bewailing the defeat of the Jews. But the High Priest Zaccaria, to instill courage into the faithful, leads in a prisoner, Fenena, daughter of Nabucco. The god of Israel will protect them and Fenena will be a precious hostage in their hands. So saying, he leaves her in the custody of the young Ismael, nephew of the king of Jerusalem, before going out at the head of the people to attempt a last defence of the city. Fenena loves Ismael, whom she rescued one day from prison when he was Jewish envoy to Babylonia. Ismael promises the girl that he will repay her generous deed. But she exhorts him to do his duty and not to risk the execration of his people for the sake of her freedom. Ismael is about to drag her away through a secret door, when Abigaille (believed by all to be the elder daughter of Nabucco) bursts into the temple, leading a band of Babylonian warriors disguised as Jews. Abigaille, who has also been in love with Ismael since she met him in Babylonia, hates Fenena. She is already planning to report her to her father for treachery, after catching her in the act of escaping with a Jewish soldier of fortune. She might be disposed to pardon the girl, she confides to Ismael – who disdainfully refuses – if he will renounce Fenena in her favour. In the meantime Levites, the people and Jewish warriors rush for shelter in the temple, terrified by the relentless advance of Nabucco at the head of the Assyrian and Babylonian armies. Hailed by Abigaille, the king rides up to the temple. Zaccaria confronts him and threatens divine wrath on his impiety. When Nabucco asks him contemptuously what forces his god can command, the priest seizes Fenena and raises his dagger. But Ismael rapidly intervenes and gives Fenena back to her father. Nabucco is once again master of the situation. His rage flares up. As he incites his troops to plunder and slaughter, Zaccaria and the people curse Ismael.
Apartment in the palace of Babylon.
Abigaille has learnt from a document, which Nabucco had kept hidden, that she is not the king’s elder daughter but a slave brought up at court, whom the Assyrians believe is heiress to the throne. Furthermore Nabucco, who is away from the capital to continue his war against the Jews, has appointed his legitimate daughter Fenena, towards whom Abigaille feels the bitter hatred of jealousy, to act as his regent. Abigaille’s ally in this dispute is the High Priest of Bel. Accompanied by the Magi and chiefs of the realm, he comes to inform her indignantly that Fenena, who is in love with Ismael, has set all the Jewish prisoners free. The time has come, he suggests, to stage a coup. A false rumour has already been put about that Nabucco has been killed in battle, and that the Babylonian people, who believe in her, will want her to be their queen. Abigaille is exultant. In a wild dream of power, she savours thoughts of revenge upon Fenena and upon Nabucco himself, and of entire populations crushed and humiliated beneath the sceptre of a slave girl.
A hall in the palace, communicated on one side with a gallery and on the other with the regent’s apartments.
Zaccaria, followed by a Levite, prepares to bring the tables of the Law to Fenena, who is soon to be converted to the Jewish religion. The «miracle» is announced by Ismael to the other Levites as they gather in the hall. But they refuse to listen to the man whom they believe to be a traitor. Zaccaria returns, however, accompanied by his sister Anna and by Fenena. He publicly declares that Ismael’s love for Nabucco’s daughter has truly converted an infidel to the religion of Abraham. So the Jews will be protected by the regent, and Ismael deserves forgiveness. Meanwhile Abdallo, an old officer in Nabucco’s service and loyal to the regent, enters, announcing the king’s death and Abigaille’s intention to seize power. He urges Fenena to escape. The girl decides instead to join the rebels to defend the legitimate throne. But she is arrested by Abigaille, surrounded by the Magi, the High Priest and a crowd of Babylonians. As Abigaille orders Fenena to relinquish the crown, Nabucco appears among his warriors, with an air of formidable majesty. He seizes the crown and places it on his head. Cursing Bel, who has made traitors of the Babylonians, and mocking the defenceless god of the Jews, he orders everyone to prostrate themselves and to worship him as the sole divine authority. Indignant at this sacrilege, Zaccaria threatens Nabucco with the punishment of heaven. Enraged by such insolence, the king orders the priest to be arrested and put to death with his people. When Fenena proudly declares that she herself is a Jewess and wishes to die with her kin, her father forces her to kneel before him and with solemn arrogance proclaims “I am no longer king, I am god”. Whereupon a flash of lightning strikes Nabucco on the head. In terror he feels the crown snatched away by a supernatural force. In anguish and bewilderment, imagining revengeful ghosts around him brandishing swords of fire, he begs help from Fenena as his strength fails him. Abigaille scornfully picks up Nabucco’s crown, determined to continue, in his stead, the struggle against the Jews.
Hanging gardens at the palace of Babylon.
The Magi, Ministers and people are gathered around a large gold statue of Bel. Abigaille is seated on the throne. The High Priest presents her with the death sentence for the Jewish prisoners and Fenena. Nabucco enters, looking ill and dishevelled. His guards step respectfully aside. Abigaille give orders for him to be led back to his rooms but Nabucco, with a glimmer of his former pride, attempts to walk up to the throne. Abigaille then dismisses the company so that she can have a decisive confrontation with the king. She reigns in his name, she tells him, summoned by the people while he lay helpless; and those people now cry death to the Jews. Nabucco consents to affix his royal seal to the parchment consigned by the High Priest to Abigaille. But he at once regrets his action, imagining Fenena’s fate. Abigaille is irremovable and dispatches the guards with the sentence sealed by the king, saying haughtily to Nabucco, that if Fenena dies, he still has another daughter. Nabucco replies angrily that she is only a slave girl. But Abigaille, drawing from her bosom the document certifying to her servile birth, tears it up before the king’s eyes. Feeble and shaky, he is not sure how to react. He calls his guards but, as Abigaille triumphantly tells him, they are no longer at his command: they have orders only to escort him as a prisoner to his rooms. Meanwhile Nabucco, his pride broken, implores Fenena’s life and is disposed permanently to renounce his throne to save her. Abigaille refuses all pity with contempt and taunts the old man for having intended to use that hidden document which she has now destroyed, to humiliate her publicly as a slave. All his belated regrets will be of no avail.
The banks of the Euphrates.
The Jews in captivity lament their unhappy fate and invoke the distant homeland of their dreams. Zaccaria reproaches them for their resignation and exhorts them to trust in a change of fortune when the lion of Judah will break their chains of servitude and come down like lightning upon the Babylonian empire.
Apartment in the palace.
Nabucco wakes from a deep torpor, imagining that he is once again leading his armies to the destruction of Jerusalem. Hearing the name Fenena repeatedly cried from outside, he looks out from the loggia and sees his daughter chained among rows of warriors, being led to execution. In despair at his powerlessness to act, he implores forgiveness from the god of the Jews, swearing to renounce his sacrilegious rites and to rebuild the Temple of Jerusalem if only he can regain his former strength, now that his mind has been enlightened by the truth. Just as he is about to push the door open violently, he is stopped by Abdallo and his faithful guards, who advise him not to go out and face humiliation from the crowd who believe he has gone mad. Proudly drawing himself up, Nabucco orders Abdallo, who joyfully obeys, to follow him: as king of Assyria he will rout the traitors, save Fenena and free the Jewish people from captivity.
The High Priest of Bel is standing in the peristyle of the temple, next to an expiatory altar on either side of which are two sacrificers armed with axes. The sound of sombre and lugubrious music announces the arrival of Fenena and the Jews condemned to the block. As Zaccaria blesses the girl who is about to win the palm of martyrdom and to rise up to heaven, Nabucco bursts in, his sword drawn. The idol of Bel crashes to the ground. All those present fall to their knees in wonder and praise the god of Israel. At that moment Abigaille, who has poisoned herself, is brought into the king’s presence by two warriors. Before dying, she asks Fenena to forgive the evil she has done her and begs Nabucco to marry the girl to Ismael. In conclusion, a consolatory Zaccaria – against all historical truth – predicts for Nabucco, as the reward for his faith in Jehovah who will protect him, dominion over all the kingdoms of the world.