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 k