Act I: Jerusalem
The Jews are praying that the Assyrian hordes who have laid siege to Jerusalem will be scattered and that the Throne of David will be freed of the impious. The high priest Zaccaria tells of hopes for their deliverance – the Jews have taken Fenena, Nabucco´s youngest daughter, and she will act as a powerful bargaining weapon in the talks with the King of Babylon.
The commander Ismaele brings news that Nabucco has launched an attack. Zaccaria wishes to entrust Ismaele with guarding Fenena, but it turns out that the young people are in love with each other… Ismaele swears he will free his beloved.
Babylonian warriors burst in, headed by Abigaille, Nabucco´s eldest daughter who is also in love with Ismaele… Abigaille proclaims Nabucco´s glory. Finally the King of Babylon himself enters. Fenena rushes to her father then to Ismaele, but Zaccaria manages to catch her and raises his dagger over her. Nabucco tries to gain time, while Abigaille hopes that now her sister and rival will die. Suddenly Ismaele snatches the dagger from Zaccaria and frees Fenena. Defeated, the Jews are overpowered by the Assyrians; they curse Ismaele.
Act II: The Impious One
Once again Nabucco departs with his warriors for Judea, leaving Fenena on the throne. Meanwhile Abigaille has managed to obtain and destroy a document which states that in fact she is not Nabucco´s daughter but the daughter of slaves. The high priest of Baal informs her that Fenena has freed the captive Jews and that conspirators have already spread news of Nabucco´s death in order to proclaim Abigaille queen.
Zaccaria proclaims a new miracle of the God of Israel – Fenena, daughter of the King of Babylon, has adopted the Jewish faith. Once again the Levites curse Ismaele, but Anna tells them that Fenena, whom he saved, is from now on one of them, and so Ismaele is pardoned. Abdallo tells of the death of Nabucco and implores Fenena to flee, but she has no intention of surrendering the throne to her sister. As Abigaille tries to seize the crown from her Nabucco appears. The Assyrian warriors again swear allegiance to him. Nabucco orders the destruction of the Jews but Fenena tries to save them, declaring that she is one of them.
Nabucco declares himself God. Heaven punishes him by driving him insane. The crown is now in Abigaille´s hands.
Act III: The Prophecy
Abigaille´s triumph. The insane Nabucco enters. Abigaille deceives him into surrendering the ring with the seal in order to ensure the death sentence on the people of Israel is carried out. Nabucco understands that this means the death of Fenena too. In an attempt to save his daughter he reminds Abigaille of her slave roots; in return Abigaille shows him the fragments of the tablet stating her true birth. Nabucco is taken into custody.
Imprisoned by the Babylonians, the Jews recall their lost homeland. Zaccaria foretells the salvation of Israel and the fall of Babylon.
Act IV: The Broken Idol
In his dungeon cell Nabucco hears cries of "Death to Fenena!" In despair he rejects the idols of Babylon and turns to the God of Israel. Together with Nabucco´s loyal warriors, Abdallo frees his king and restores his crown.
The Jews are awaiting execution. Nabucco appears, proclaiming the God of Israel. Coming together, the Jews and Assyrians join him. Abigaille has taken some poison. With her dying words she begs forgiveness of Fenena and turns to Jehovah.