Scene 1. Solomon Temple in Jerusalem.

Babylonian army surrounds Jerusalem. The priests, levites and common citizens pray and ask God to protect them from cruel Babylonians. The High Priest Zaccaria tells people not to despair for he managed to take Fenena, the daughter of the Babylonian king, a prisoner. Such a captive may bring peace between Assyrian and Jewish people.

The nephew of the king of Jerusalem Ismaele recognizes his beloved Fenena who once has released him from the captivity in Babylon. The young man wishes to save the princess. Meanwhile Abigaille, the natural daughter of Nabucco, bursts in leading Assyrian soldiers to occupy the temple. She desires to inherit the throne. She is passionately in love with Ismaele but her love is unrequited. Abigaille promises to save Jewish people in return of Ismaele love but it is impossible. The temple is overcrowded. Assyrian army is approaching Jerusalem. Nabucco appears. Zaccaria confronts him and threatens to stab Fenena if the Assyrian king and his warriors desecrate the temple. But Ismaele holds back Zaccaria’s blow and delivers Fenena to her father. While Zaccaria and the others curse Ismaele, Nabucco orders to loot and burn the temple.

Scene 2. The royal apartments in Babylon

Abigaille has obtained the document confirming that she is a daughter of the king and the slave-girl. This means that she cannot inherit the throne. The High Priest enters and informs Abigaille that Nabucco has appointed Fenena the regent for the time of his absence. But she has released the Jewish captives and wants to be Judaized. The High Priest decides to offer Abigaille the throne and tells people that their king was killed in the battle.

Being captured Zaccaria prays for help to his people. Ismaele comes to levites and asks for forgiveness but they accuse him of committing high treason. Zaccaria’s sister Anna appears and declares that Fenena will help the Jewish people.

Having heard joyful news people come to the palace. Abdallo, a soldier, comes and tells Fenena that priest and Abigaille are going to arrest Fenena as a traitress. Abdallo suggests Fenena to escape but she refuses. Abigaille demands the royal scepter. At that moment, to the astonishment of all people, Nabucco enters, takes the crown and puts it on his head. He is irritated by the treachery and proclaims himself a king and a god. Suddenly lightning strikes him and he becomes insane. Abigaille takes the crown.

Scene 3. The Throne room in the palace in Babylon

Abigaille is the Queen of Babylon now. The ball is held in the palace in her honor. The High Priest asks her to put the Jews and Fenena to death. Suddenly Nabucco appears. He is insane and is under house arrest. Abigaille dismisses the others and makes him to sign the death warrant for Jewish people and his own daughter. Nabucco is overcome with grief and anger having realized what he had done. He tries to find the document that proves Abigaille’s slave origin. But that is Abigaille who has is and she destroys it triumphantly. Nabucco pleads with Abigaille for Fenena’s life, but she refuses.

Scene 4. The bank of the River Euphrates

The Jews are desperately homesick being so far from their motherland. Zaccaria appears and predicts that Babylon will fall and God will help them to be released from captivity.

Scene 5. The royal palace in Babylon

Imprisoned Nabucco awakens in the palace, after the nightmares. He sees Fenena waiting for the execution. He prays to God of the Jews for forgiveness and asks to save his daughter from death. His strength and reason are immediately restored. Nabucco with his faithful followers goes to save Fenena. The statue of Assyrian god falls before the eyes of astonished people. Nabucco glorifies Jehovah and frees the prisoners. Abigaille loses last hope to come to power and takes poison. Praying and begging pardon she dies.