Act One:

King Nabucco and his army of Babylonians are defeating the Jews and poised to break through the gates of Jerusalem. Inside the Temple of Solomon, Zaccaria, the high priest of the Hebrews, urges his people to trust in God, reminding them that peace is still possible. The Jews are holding Nabucco's younger daughter, Fenena, hostage in hopes that Nabucco will promise peace in exchange for her life. Ismaele, nephew to the King of Jerusalem and former envoy to Babylon, enters with his soldiers to announce Nabucco is close by.

Zaccaria entrusts custody of Fenena to Ismaele, not knowing that the two are in love. Upon Zaccaria's departure, Ismaele urges Fenena to escape. Before she can make a move, Fenena's older sister, Abigaille, leading a platoon of soldiers, storms in and discovers the lovers. Abigaille is dressed as a warrior and her men are disguised as Hebrew soldiers. Abigaille, who is also in love with Ismaele, offers him a deal: return her love and his people will be spared. If he refuses the offer, Fenena will be accused of treason. Ismaele refuses the deal and offers his life in exchange for the safety of his people. Before Abigaille can give an answer, Nabucco and his men storm the temple. Zaccaria, who has returned to the temple, tries to stop Nabucco by threatening to stab Fenena to death. But, Ismaele steps in and returns Fenena to her father, infuriating the Jews. Nabucco orders the temple destroyed and leaves. The Jews curse Ismaele as a traitor.

Act Two:

King Nabucco has appointed Fenena as Regent of Jerusalem, and she has converted to the Judaism, which prompts the Jews to accept Ismaele back into the city.) Abigaille has discovered a secret document alleging she is actually a slave, not Nabucco's daughter. The High Priest of Baal and the Magi reveal to Abigaille that Fenena is now releasing the Jewish prisoners. Abigaille, the High Priest and the Magi conspire to seize the throne. An old officer of the king warns Fenena to flee but there is no time. Abigaille falsely announces that Nabucco died in battle and she demands the crown from Fenena. Unexpectedly, Nabucco returns to the palace and claims the crown. He also proclaims himself a god having overthrown Baal and Jehovah.

When he demands that Zaccaria and Fenena kneel before him, he is struck by lightning from the God of the Jews, knocking the sense out of him and the crown off his head. Abigaille picks it up and puts it on.

Act Three:

In the Hanging Gardens of Babylon, the High Priest of Baal gives Abigaille the death decree for the Jews. Nabucco wanders into the gardens and tries to reclaim the throne, though he's lost his mind. Abigaille dismisses everyone to have a private discussion with Nabucco. She explains that, as he's not well, she's acting as regent. She then talks him into signing the decree. In a moment of clarity, Nabucco realizes he's sealed the fate of Fenena and pleads for her life. When Abigaille rejects the plea, Nabucco tells her that she is not really his daughter, but a slave. Abigaille mocks him, destroying the document that confirms her true origins, and declares herself the only daughter and heir to the throne. Meanwhile, on the banks of the Euphrates River, during a short break from forced labor, the Jews reminisce of their homeland. Zaccaria asks them to have faith that God will destroy Babylon.

Act Four:

King Nabucco wakes up with his reason restored, but, now, as Abigaille's prisoner. When he sees Fenena and the Jews being taken to the sacrificial altar of Baal, Nabucco kneels and prays to God for forgiveness. He promises to rebuild the Temple of Solomon and follow the true faith. Realizing Nabucco is back to his old self, a few loyal soldiers free him from prison to lead a revolt. They rush to the Hanging Gardens, where Nabucco rescues Fenena, shatters the idol of Baal and tells the Jews that they are free. He promises that a new temple will be raised to their God. Abigaille appears, but has just poisoned herself. She asks God for forgiveness, urges Fenena to marry Ismaele, and dies. Zaccaria acclaims Nabucco as the servant of God and the King of Kings.