Jerusalem: Assyria and Israel are at war with one another. Nabucco, the King of the Assyrians, has occupied Jerusalem. The last surviving Israelites - Levites and temple attendants - have fled to the sanctuary of the temple. The prophet Zaccaria takes a
hostage with him: Nabucco's daughter Fenena, who once helped the Jew Ismaele to escape from Babylon, where Fenena's sister Abigaille had held him captive out of jealousy. Ismaele is surprised by Abigaille as he is trying to help Fenena to escape. Abigaille tries to take advantage of Ismaele: only his love for her can save the Jews. However, Ismaele decides for Fenena. When Nabucco enters the temple, Zaccaria is about to kill his hostage when Ismaele seizes his arm to save Fenena. His people curse
him, and all the Hebrews are taken captive.
The Impious One: Ismaele lives in freedom under the curse. Nabucco has passed his crown to Fenena during his absence. The high priest of the Babylonians passes Abigaille a secret document giving the reasons for her discrimination: she is the
daughter of a slave. This means that Abigaille is susceptible to blackmail and can be utilized for the high priest's aspirations of power. He offers her the crown; the priests have already spread the rumour of Nabucco's death. In order to break the curse on
Ismaele, Fenena has released the Jews, but they remain remorseless. By converting to the Jewish faith, Fenena makes herself guilty of high treason - exactly in accordance with the high priest's plan. The two sisters are now rivals for power, but the decisive struggle is waged behind the scenes by Zaccaria and the high priest. On his return Nabucco realizes that he has been betrayed on all sides and curses the faith of both Babylonians and Jews. Nabucco declares himself to be the god of the Babylonians - and Fenena is to be his first sacrifice. Such arrogance and abuse of power causes Nabucco to lose his senses.
The Prophecy: Abigaille is now able to revel in the glory of being queen: all the Hebrews are once again in captivity - including her beloved Ismaele. The high priest calls for them all to be annihilated. Made clairvoyant by his insanity, Nabucco sees through Abigaille's plans. Father and daughter now blackmail one another: he calls her a slave, and she plays on his powerlessness as a king. Nabucco is not yet capable of fathoming the fate of an entire people, and merely begs for mercy for Fenena. But in vain. After he has signed the death sentence, he is himself taken prisoner. The imprisoned Jews are awaiting their death. In a bloody vision Zaccaria prophesies the downfall and destruction of all the enemies of Israel.
The Broken Idol: Nabucco has been worn-down by these terrible events. He deplores his entire life, a life of genocide, even murdering his own daughter. Suddenly, the doors of the prison burst open. Nabucco and his soldiers are able to prevent the execution of the Jews and free Fenena. But help comes too late for Abigaille, who has put an end to her own life. The King of the Assyrians grants everyone their freedom. At last there
may now be peace between Assyria and Israel...