PART 1: JERUSALEM
The Temple of Solomon. Fleeing the approaching army of the Assyrian king Nabucco, the Israelites have taken refuge in the Temple. Zaccaria, their high priest, tries to quiet the crowd. He holds valuable collateral: Nabucco’s favorite daughter Fenena, who has been at the Temple as an intermediary. Fenena has a relationship with the Jewish prince Ismaele, is fascinated by the Jewish faith, and seeks reconciliation between the enemy nations. Fenena and Ismaele are surprised by Abigaille, who is supposedly Fenena’s older sister. Abigaille, too, has fallen in love with Ismaele. She offers to spare his people if he returns her love, but he chooses to remain faithful to Fenena.
Nabucco enters the Temple with the aid of traitors. In an attempt to hinder Nabucco’s work of destruction, Zaccaria takes Fenena hostage, threatening her with a dagger, but Ismaele frees her. Nabucco orders the Temple looted and burned. The surviving Jews are taken to Babylon in chains.
PART 2: THE IMPIOUS ONE
Nabucco’s palace in Babylon. Nabucco has appointed Fenena regent for the duration of his campaign. Abigaille, who feels slighted, also discovers that she is not Nabucco’s natural daughter, but rather the offspring of her mother’s adulterous affair with a slave. The High Priest of Baal then informs her that Fenena has released the Jewish prisoners who have been brought as slaves to Babylon. He therefore intends to install Abigaille as successor to the throne and has spread a rumor that Nabucco has fallen in battle. With Abigaille as ruler, the priest hopes to advance his own interests.
Zaccaria arranges for Fenena to convert to the Jewish faith. Ismaele encounters the Levites, who curse him for having brought about Israel’s downfall by freeing Fenena. But Zaccaria and his sister Anna announce that the conversion of the rescued princess retroactively justifies Ismaele’s deed. Abigaille forces her way in and demands that Fenena surrender the crown. But the reportedly dead Nabucco intercedes, shielding Fenena and declaring the religion of the rebellious Babylonian priests abolished. When Fenena reveals to him that she is now one of the people of Jehovah, he declares himself the one and only God. At that moment he is struck by lightning. Abigaille crowns herself as the new queen.
PART 3: THE PROPHECY
The High Priest of Baal has drawn up an edict ordering the execution of all the Jews. Abigaille, now queen, hesitates to sign it. Nabucco enters. Enfeebled and disheveled, he confronts Abigaille and denies her right to the throne. Abigaille claims only to be acting in his stead. Arguing that sparing the Jews would be a confession of his weakness, she succeeds in persuading him to sign. Too late he realizes that by doing so he would also deliver Fenena to the executioner. Abigaille offers herself to him as a replacement for his lost daughter. But Nabucco hurls the secret of her illegitimate origin back in her face and pleads only for mercy for Fenena. Abigaille realizes that her stepfather’s love will never be hers.
The Israelites long for their lost homeland. Zaccaria prophesies Babylon’s ruin.
PART 4: THE BROKEN IDOL
Nabucco raves incoherently. When he hears the fanfares announcing Fenena’s execution, he begs the God of the Jews to forgive him for his pride. His prayer is heard; Nabucco is restored. His abider Abdallo and his soldiers hurry to his side.
Nabucco manages to stop Fenena’s execution and exhorts the crowd to destroy the idol of Baal, which shatters of its own accord. Nabucco sets the Israelites free with the order to build a new Temple. All praise Jehovah. Even the dying Abigaille, who has taken poison, prays to the God of the Jews for forgiveness.