Ancient Jerusalem and Babylon, 587 BCE
Part 1: Jerusalem
The Temple of Solomon in Jerusalem. The Hebrews, that is, the people of Judah, pray as the armies of Babylon, led by King Nabucco (Nebuchadnezzar), sack their city. The High Priest, Zaccaria, urges the Hebrews not to despair:they have captured Nabucco's daughter, Fenena, and are holding her hostage. Zaccaria entrusts her to Ismaele, nephew of the king of Judah, not knowing that she and Ismaele are in love. (Earlier, when Ismaele was captive in Babylon, Fenena had fallen in love with him and helped him escape.) Abigaille, Fenena's half-sister, surprises the lovers. She, too, loves Ismaele; but he rebuffs her. Suddenly, Nabucco and his army march into the temple. Zaccaria confronts Nabucco and threatens to kill Fenena, but Ismaele protects his beloved. The Hebrews curse Ismaele as a traitor, and Nabucco orders the destruction of the temple.
Part 2: The Blasphemer
Nabucco's palace in Babylon. Abigaille has found a document proving that Nabucco is not really her father;that she is in fact lower than a slave. She intends to conceal the document and destroy Nabucco and Fenena. The High Priest of Baal (the Babylonian god) informs Abigaille that Fenena, appointed regent while Nabucco wages war, has freed Ismaele and the Hebrew captives. Dismayed by Fenena's treason, the priest suggests that Abigaille should be the one to rule Babylon. He has already begun spreading the rumor that Nabucco has fallen in battle. Abigaille vows to seize the throne.
Among the captive Hebrews, Zaccaria reads over the Tablets of Law, praying for inspiration to persuade the Babylonians to renounce their false idols. The Hebrews are furious with Ismaele for sparing Fenena, but they soften when they discover that she has converted to Judaism.
The rumor that Nabucco is dead reaches the palace. Supported by the Babylonians, Abigaille demands the crown from Fenena. But at that moment Nabucco himself enters. He declares that he is no longer king—he is god. A thunderbolt strikes him down; the crown falls from his head and he goes mad. Abigaille grabs the crown and proclaims herself ruler of Babylon.
Part 3: The Prophecy
In the Hanging Gardens of Babylon. The High Priest urges Abigaille to have the Hebrews put to death. Before she can give the order, the mad Nabucco wanders in. Abigaille tricks him into signing the death warrant for the Hebrews. He finds out too late that he has consigned Fenena, now a Jew, to be killed. Abigaille tries to remind Nabucco that she, too, is his daughter, whereupon Nabucco looks for the document proving that Abigaille is the daughter of a slave. She rips the paper to pieces. Realizing his powerlessness, Nabucco pleads in vain for Fenena's life.
On the banks of the Euphrates. A chain-gang of enslaved Hebrews rests from forced labor, and in the great chorus "Va, pensiero," their thoughts turn to their beloved Zion. Zaccaria inspires them with a prophecy of the destruction of Babylon and the holy fire of courage.
Part 4: The Shattered Idol
From a window Nabucco watches Fenena and the Hebrew slaves being led to execution. Desperate, he prays to the God of the Hebrews for forgiveness. He promises to rebuild the temple in Jerusalem and convert to Judaism. His strength and reason are restored; he opens the door of the room where he is locked, gathers a band of loyal soldiers, and resolves to regain his throne, punish the traitors, and save his daughter.
The Hebrews are about to be sacrificed to Baal. Fenena serenely prepares for death when Nabucco rushes in and stops the sacrifice. He declares he will rebuild the Temple of Jerusalem and worship the God of the Hebrews. All join in prayer. Abigaille confesses her crimes, begs forgiveness of Fenena, prays for God's mercy, and dies. Nabucco frees the Hebrews, and everyone unites in praising God.