Act I. Jerusalem
The Temple of Solomon, Jerusalem, 587 B.C. Zaccaria, high priest of the Israelites, consoles his people after their defeat by the Babylonian army, led by King Nabucco. The Hebrews have captured Nabucco’s daughter, Fenena. Zaccaria entrusts her to Ismaele, nephew of the king of Jerusalem, unaware that they are lovers. They are planning to flee when they are discovered by Abigaille, Nabucco’s eldest daughter. She too, is in love with Ismaele, and offers him the freedom of his people if he will return her love, but the young man refuses. Meanwhile, the Hebrews take refuge in the temple, fleeing from Nabucco. Zaccaria uses Fenena as a hostage to prevent Nabucco entering the sacred temple, threatening to kill her with a dagger. But her lover, Ismaele, saves her to the amazement of his fellow Israelites, who accuse him of being a traitor.
Act II. The Impious One
The palace of Nabucco in Babylon. Abigaille finds a scroll revealing that she is not the daughter of Nabucco, but of slaves. With the support of the high priest of Baal, she attempts to seize the throne in the absence of Nabucco, who is believed to have died on the battlefield. But just as she prepares to take the crown from Fenena, who Zaccaria has converted to Judaism, Nabucco bursts in and stops her. Furious with both the Hebrews and the Babylonians, he declares himself their only god. At this, a ray of divine light strikes the crown from his head. Nabucco loses his mind, and Abigaille retrieves the crown for herself.
Act III. The Prophecy
Abigaille tricks a still raving Nabucco to sign a death warrant for all the Israelites. He reveals to her that she is not his daughter. Abigaille is unmoved; she mocks the man she had thought to be her father by producing the document proving her low birth and tearing it to pieces before his eyes. Nabucco begs her to at least spare his daughter’s life. Meanwhile, on the banks of the Euphrates, the enslaved Hebrews yearn to return one day to their “beautiful and lost” homeland.
Act IV. The Broken Idol
Nabucco returns to his senses and finds out that Fenena is to be sacrificed at the altar of Baal. After turning to the god of the Hebrews and recognising Him to be the true god, he seizes power back from Abigaille and frees the prisoners, allowing them to return home and promising to build them a new temple in Jerusalem. Abigaille chooses to end her own life by taking poison, and in her dying moments, begs forgiveness of Nabucco and Fenena.