In Solomon’s temple, the Hebrew people implore the protection of their God against the Assyrian invasion led by Nabucco, king of Babylon. The high priest Zaccaria urges them to fight and tells them that he has taken captive Fenena, Nabucco’s daughter, whom he intends to use to restore peace. lsmaele, nephew of the King of Jerusalem, arrives with disastrous news: the enemy troops have gained more ground. But Zaccaria remains confident. He hands Fenena over to lsmaele before leaving, at the head of the Hebrews, to defend the temple. Left alone, lsmaele and Fenena no longer must conceal their tender feelings. He reminds her how she helped him escape from a Babylonian prison when he was an emissary to Nabucco’s court, and now wants to return the favor and run away with her. They are interrupted by Abigaille, Fenena’s sister: Jerusalem has fallen, the Hebrews are defeated. Abigaille, also in love with lsmaele, offers to save his people if he gives her his heart. lsmaele refuses and Fenena, overwhelmed by such heroism, suddenly converts to the God of the Hebrews. The people return in panic, preceding a triumphant Nabucco. Cornered, Zaccaria threatens to execute Fenena, who implores her father to have mercy on his enemies. But it is lsmaele who stands between her and the knife and saves his beloved. Nabucco’s anger can then be unleashed: he orders the destruction of the temple and the enslavement of the Hebrews, while Zaccaria curses lsmaele, traitor to his people.
In the Royal Palace in Babylon, Abigaille has just made a discovery: a document proves that she is not really the daughter of Nabucco but a child of slaves. The High Priest of Baal enters and informs Abigaille that Fenena is about to free the Hebrew slaves, it is therefore time to accelerate the plan that will put Abigaille on the throne. To do this, he has already spread the rumor of Nabucco’s death on the battlefield.
Elsewhere in the palace, Zaccaria pleads with God to inspire him to free his people, while lsmaele, an outcast among his own people, must answer for his treachery. lt is Anna, Zaccaria’s sister, who justifies him: Fenena having converted to the God of the Hebrews, lsmaele has not saved an enemy but one of their own. Abdallo, Nabucco’s faithful officer, urges Fenena to flee. The rumor of the king’s death has led the Babylonians to rally behind Abigaille, supported by the High Priest. She refuses to return the crown to Fenena when Nabucco appears and angrily states that from now on, he will not tolerate either the God of the Hebrews or those of the Babylonians: all must revere only him. He also condemns the Hebrews to death, prompting Fenena to reveal her conversion. A bolt of lightning then strikes Nabucco, knocking the crown off his head and driving him mad. Cries out for divine punishment and Abigaille seizes the crown.
In the Hanging Gardens of Babylon, Abigaille sits on her throne by the statue of the god Baal and receives the tributes of her subjects. The High Priest asks her to hasten the slaughter of the Hebrews and Fenena. Abigaille pretends to hesitate, when Nabucco enters, disoriented, with shaggy beard and tattered clothes. Abigaille treats him with contempt, then tells him to sign the deed of condemnation. Nabucco signs but asks about the fate of his daughter. Abigaille replies that no one can save Fenena, prompting Nabucco to threaten to revea her condition as a slave through a document. But Abigaille tears it to pieces in front of Nabucco’s eyes as trumpets sound announcing the imminent execution of the Hebrews. Nabucco tries one last time to save Fenena, offering Abigaille the throne in exchange for his daughter’s life. But the new queen disdainfully refuses and has this ‘shadow of a king’ arrested.
On the banks of the Euphrates, the enslaved and enchained Hebrews sing of their lost homeland, which they visit in thought for want of being able to return there. But Zaccaria urges them to stop lamenting and to rise again. He prophesies that their enemies will soon experience the fury of the Lion of Judah and that Babylon will be destroyed to the last stone.
In the palace, Nabucco is awakened from a nightmare by cries that he takes as a call to war. But it is in fact the noise of the people calling for Fenena’s death. Imprisoned and powerless, Nabucco turns to the God of the Hebrews and asks for forgiveness. lf he saves his daughter, he will acknowledge Him as the true God and rebuild the temple in Jerusalem. Abdallo arrives miraculously, seeing that the king has regained his sanity, gives his sword and together with a troop of soldiers he declares himself ready to help him regain the throne.
In the gardens, the Hebrews prepare to be slaughtered. As Zaccaria represents martyrdom to Fenena at the end of their exile and the High Priest is about to begin the sacrifice, Nabucco bursts in at the head of his troops. The statue of the god Baal falls and shatters. Nabucco announces that he is freeing the Hebrews, that the temple in Jerusalem will be rebuilt and that Abigaille, with a troubled mind, has poisoned herself. AII are proclaiming the greatness of the God of the Hebrews when two soldiers enter, supporting the dying Abigaille. She asks Fenena, lsmaele and Nabucco for forgiveness, implores the God who ‘lifts up the afflicted’ and converts to Him before breathing her last. Zaccaria then declares that Nabucco, by serving Jehovah, will be King of Kings.
