The Temple in jerusalem
The priests and people of Jerusalem are gathered in the refuge of their Temple, offering up desperate prayers that the Temple will be saved from the wrath of the invading Nabucco, King of Babylon. Zaccaria, the High Priest of Jerusalem, urges the people to put their faith in God. At that moment, Ismaele, the nephew of the King of Jerusalem, bursts into the Temple with the news that Nabucco and his forces are advancing rapidly. As the people shrink back in terror, Zaccaria entrusts the prisoner Fenena, the daughter of Nabucco, to Ismaele’s care.
Left alone with Fenena, whom he has loved ever since she rescued him from her father’s prison when he was the Jewish envoy to Babylon, Ismaele vows that he still loves her and will save her. As he begins to lead her to freedom, Abigaille, a slave who is reputed to be the eldest daughter of Nabucco, enters the Temple with a band of Babylonian soldiers disguised as Hebrews. Abigaille, who also loves Ismaele, threatens the two with death but tells Ismaele that he can save his own life by returning her love. He refuses, as Zaccaria and the other Hebrews rush back into the Temple with the news that Nabucco and his men are almost at the Temple door. Zaccaria demands to know how Nabucco and his forces have broken through so quickly, and Ismaele points to Abigaille and her soldiers, explaining that they came in disguise. Nabucco himself enters, but Zaccaria bars his way, warning that Nabucco shall not profane the Temple of God. As Nabucco pushes his way in, Zaccaria seizes Fenena, and threatens to kill her with his dagger if Nabucco persists. Nabucco commands Zaccaria to yield, but as Zaccaria moves to stab Fenena, Ismaele wrests the dagger away from him and sets Fenena free. Zaccaria curses Ismaele for his “treachery,” as Nabucco orders his men to sack and burn the Temple and murder the inhabitants.
The royal apartments in babylon
The Hebrews have been imprisoned in Babylon, and Fenena has been installed as Regent while Nabucco leads his soldiers on other conquests. Abigaille, burning with jealousy over Fenena’s elevation to the throne and her hold on Ismaele’s love, seeks proof that she, Abigaille, is indeed Nabucco’s legitimate daughter and heir. She finds a document proving instead that she is not Nabucco’s daughter at all, but the child of slaves. The High Priest of Baal then tells her that Fenena plans to free the Hebrew prisoners, and urges Abigaille to seize the throne and put the Hebrews to death. He reveals that he has already spread a rumor that Nabucco has died in battle, thus leaving the throne open for Abigaille.
A palace chamber and a hall in the palace
The Hebrews are gathered in prayer and Zaccaria tells them that he has been commanded by God to be His agent in the destruction of Babylon. The people curse Ismaele for saving the life of Fenena, but Zaccaria reveals that Fenena has converted to the Hebrew faith. As the Hebrews rejoice, Abdallo, an old retainer of Nabucco, rushes in with the news that Nabucco is dead and warns that the people are hailing Abigaille as their Queen and calling for the death of the Hebrew prisoners.
Abigaille and the High Priest of Baal enter, demanding that Fenena yield the crown to Abigaille. As the two women confront one another, Nabucco storms into the room, seizes the crown and places it on his head and pronounces himself not only the King, but God. He commands Zaccaria, Fenena and the others to prostrate themselves before him, but they refuse, aghast at his blasphemy. At that moment, a thunderbolt crashes the crown from Nabucco’s head as the people gasp in shock. Nabucco, driven temporarily insane, bewails his fate but Abigaille retrieves the fallen crown and vows that the greatness of Babylon shall continue.
The royal hall
Abigaille, who has taken the throne, is holding court when the disheveled and distracted Nabucco wanders in to the throne room. Seeing Abigaille on his throne, Nabucco commands her to step down, but she dismisses her attendants and proceeds to trick Nabucco into signing the death warrants of the Hebrew prisoners. Once he has signed, she tears up the document which proves her parentage, and tells Nabucco that she reigns now and that he is but a prisoner. When he realizes that he cannot stop her, Nabucco begs for her mercy, but Abigaille merely mocks him.
The banks of the euphrates
The Hebrew prisoners, singing the famous chorus “Va, pensiero”, are lamenting their enslavement and the loss of their homeland. Zaccaria chastises them for their lack of faith, and tells them God will soon strike down their Babylonian captors.
The prison
Nabucco, held prisoner in his own palace by Abigaille, wakens from a nightmare. Hearing his daughter Fenena’s name shouted outside, he looks out and sees her being led to her execution by Abigaille’s soldiers. Falling to his knees, Nabucco prays to God for forgiveness and vows to worship God and banish the false idols. He tries to escape and the faithful Abdallo, with some of Nabucco’s loyal soldiers, opens the door for him. Seeing that Nabucco is rational again, Abdallo hands him his sword and they rush out to rescue Fenena.
The great hall
Before the idols of Babylon, Abigaille and her followers watch as Fenena and the Hebrews prepare for death. Suddenly, shouts of “Long live Nabucco” are heard, and Nabucco and his followers rush in. He orders the false idols swept to the ground and proclaims the glory of God. As the Hebrews rejoice, Abigaille, who has taken poison, is brought forward, dying. She begs the forgiveness of God and Nabucco with her final breath. Zaccaria blesses Nabucco, and pronounces him King of God’s kingdom on earth.