ACT I: Jerusalem
Thus saith the Lord: Behold I will give this city into the hands of the king of Babylon: he will burn it with fire.
The interior of the Temple of Solomon
The Hebrews are taking refuge in the temple from the wrath of the Assyrians, who are advancing under the command of their king, Nabucco (Nebuchadnezzar). As they are praying for their danger to be averted, the high priest, Zaccaria, brings in a captive, Fenena, the daughter of Nabucco, who, he says, will bring them peace.
Ismaele, the nephew of the king of Jerusalem, comes in with further news of the advance of Nabucco, who is now close at hand. Uttering a prayer that heaven may yet strike him down and save them, Zaccaria leaves, followed by his flock, entrusting Fenena to the charge of Ismaele. They are then able to speak of their love for each other, dating from Ismaele's visit to Babylon as ambassador, where Fenena had rescued him from prison and from the impassioned advances of her sister Abigaille.
The reunion is interrupted by Abigaille herself, sword in hand, with an advance band of Assyrians who overpower the Hebrew guards and disguise themselves in their unifroms. Abigaille upbraids Fenena for her lack of patriotism in loving one of the enemy and then falls into the same fault herself, telling Ismaele that she can still save him if he will love her. Naturally he rejects her advances.
The confrontation is ended by the inrushing Hebrews, full of alarm at the destruction outside and hoping to find refuge in the temple - to no avail, as Nabucco strides in with his war-band, undeterred by Zaccaria's warning that this is the house of God. Zaccaria makes it clear how he intends to use Fenena as a peacemaker by threatening to kill her.
Nabucco is only momentarily restrained by this challenge, but when Zaccaria shows that he intends to carry out his theatre, the dagger is struck from his hand by Ismaele who is immediately overcome by remorse as well as being overwhelmed by the curses of the Hebrews. The advantage thus rests with the Assyrians.
ACT II: The Wicked Man
Behold the whirlwind of the Lord goeth forth with fury; it shall fall upon the head of the wicked.
Scene 1: The royal apartments in Nabucco's palace in Babylon
Abigaille enters with a secret document she has managed to steal from Nabucco. It reveals that she is not, as she had been brought up to believe, the elder daughter of Nabucco, but of slave birth. She rages at this and at the fact that Fenena has been made regent while Nabucco is absent fighting.
The high priest of Bel brings the news that Fenena is setting free the Hebrews, who had been carried off into captivity in Babylon after the fall of Jerusalem. They strike up an alliance. The priest has already spread the rumor that Nabucco has been killed in battle and is prepared to support Abigaille in her attempt to win the throne.
Scene 2: A hall in the palace
Zaccaria, entering with a Levite carrying the Tables of the Law, pauses to pray. He reveals that he has a mission from God to convert a heathen soul and casts fulminations on the people of Babylon.
When he has gone, the rest of the Levites assemble, waiting to find out why they have been summoned. When Ismaele joins them they turn on him with imprecations for his treachery. His prayers for mercy fall on deaf ears until Anna, the sister of Zaccaria, followed by Zaccaria himself, brings the news that Fenena is now converted to the Hebrew faith, so Ismaele can be forgiven for saving her life.
Abdallo, the loyal retainer of Nabucco, rushes in with the news that Nabucco has been killed and that Abigaille is being acclaimed by the people. Abigaille appears and demands the crown, which Fenena is wearing.
Before this confrontation can develop further, Nabucco himself, reports of whose death have been greatly exaggerated, enters and seizes the crown himself. He proclaims himself not only still king, but also God. He is struck down by a thunderbolt, and goes mad on the spot. Abigaille picks up the fallen crown and claims it
ACT III: The Prophecy
The wild beasts of the desert shall dwell in Babylon, together with owls and hoopoes shall dwell therein.
Scene 1: The Hanging Gardens
Abigaille has consolidated her position on the throne and, in consultation with the priests of Bel, is about to order the execution of the Hebrews and of Fenena. Nabucco, still mad, dishevelled and dressed in rags, breaks through the guards and confronts her. In answer to his rage at finding her on the throne she answers that he is not well and then tricks him into signing the death warrant.
When he realises that he has been made to condemn his own daughter to death he tries to threaten Abigaille with the paper proving her low birth. But she has already stolen it and produces it only to tear it to pieces before his eyes. She mocks his entreaties for Fenena's life and threatens him with imprisonment.
Scene 2. The banks of the Euphrates
The Hebrews, working as slaves, lament their exiled state. They are joined by Zaccaria who upbraids them for their weakness and stirs them with the stern prophecy that Babylon the proud will soon fall and not a stone will be left. Their spirits are raised.
ACT IV: The Broken Idol
Bel is confounded: his idols are broken in pieces.
Scene 1: An apartment in the palace
Nabucco has been confined by order of Abigaille. He is still mad. Hearing shouts outside announcing that Fenena is about to be executed he looks out and sees a procession of Hebrews, with his daughter, being led off in chains. Unable to burst out of his prison he turns to the God of the Hebrews, praying for forgiveness and promising to cast down the false idols of Babylon.
The faithful Abdallo comes in and discovers, to his joy, that Nabucco has gone sane. He restores his sword to his master and they set off to rescue Fenena.
Scene 2: The Hanging Gardens
The high priest of Bel is ready beside his sacrificial altar when Fenena and the Hebrews are led in. Fenena kneels before Zaccaria and he gives her his blessing. She is preparing calmly for her death when Nabucco, at the head of his loyal soldiers, bursts in and orders the idols of Bel to be pulled down.
Before anyone can move the idol crashes to the ground. Nabucco proclaims the might of Jehovah, sets the Hebrews free and promises them a safe return to their own land.
As further proof of the power of Jehovah he informs them that Abigaille has been so struck by the wickedness of her ways that she has taken poison. She staggers in, dying, in penitential black, full of remorse and begging forgiveness all round. She gives her blessing to the marriage of Fenena and Ismaele and dies, praying to God for mercy.
Nabucco and the Hebrews together proclaim the might of God and Zaccaria promises greatness to the newly-converted Nabucco.