PART ONE: Jerusalem: Set in 586 B.C., the opera begins at the temple of Solomon in Jerusalem, and from there moves to the palaces of Babylon. As things get started, Jerusalem is on the verge of capture by the Babylonians and their king, Nebuchadnezzar — in Italian, he's called Nabucco. The Hebrews have taken refuge in the temple, where the high priest Zaccaria cheers his people by producing a Babylonian prisoner, Nabucco's daughter Fenena.
When the crowd breaks up, Fenena is left with Ismaele, a nephew of the King of Jerusalem. While they're alone together, a group of Babylonian soldiers charges in, led by Abigaille, Fenena's older sister, whose forces have captured the temple. Just before Abigaille's furious appearance, we've learned that Fenena and Ismaele are in love — they first met in Babylon, where Fenena helped him escape from prison. But she also helped him escape from Abigaille, who had her own designs on Ismaele, and still does. Now, Abigaille is not amused at finding Ismaele in cozy quarters with her little sister. But when Abigaille offers him freedom in exchange for his love, Ismaele is unimpressed.
As the Jews return to the temple, led by Zaccaria, Nabucco also rides up, in triumph. There's a violent confrontation and Zaccaria grabs Fenena, threatening to kill her. But Ismaele intervenes, and Fenena runs to her father. As Nabucco orders his army to sack the temple, the Jews denounce Ismaele for saving Fenena.
PART TWO: The Impious One: The Jews have been taken captive and exiled to Babylon, where an unhappy Abigaille is alone in the palace. Nabucco has left for battle, and instead of leaving Abigaille on the throne, he's passed her over and left little sister Fenena in charge — and Abigaille has just discovered why. She's found a document proving that she is not Nabucco's natural daughter; she was born a slave, and adopted by the king as an infant. The High Priest of Baal enters and urges her to seize the throne herself. He then helps out by spreading the word that Nabucco is dead.
In another room in the palace, Zaccaria reveals to his people that Fenena, who sits on the throne of Babyon, has converted, and is now a Jew.
News of the war then arrives, and it's not good for the Babylonians; there are reports that Nabucco has been killed. Abigaille uses that news to make her own bid for power. She's just putting on the royal crown when Nabucco — who isn't dead after all — makes a dramatic return, and takes it from her.
The scene builds as the various factions and individuals declare their intentions. Finally, Nabucco declares that he's not only the king, but he is also a god on earth. Suddenly there's a clap of thunder. The crown is miraculously thrown from Nabucco's head and dashed to the floor and Nabucco instantly goes mad. With the king babbling incoherently, Abigaille decides that Babylon is rightfully hers, and finds little opposition.
PART THREE: The Prophecy: The setting is still Babylon, at the legendary hanging gardens. Nabucco is still the king, but in name only. Abigaille has taken power by dethroning Fenena. The High Priest of Baal has announced that Fenena's behavior is treasonous, and decrees that she must be executed along with all the captured Hebrews.
After an opening chorus, and the fateful declaration by the High Priest, the first scene is dominated by a tense encounter between Abigaille and Nabucco. Legally, he's still the king. That means Fenena and the Jews can only be killed by his command, which he refuses to give. Then, when Abigaille tricks Nabucco into signing the order, he decides to play his trump card. Nabucco reveals Abigaille's true origins, as a slave girl he adopted in infancy. But Abigaille already knows about that — and she has destroyed the only document that proves it. Nabucco is reduced to pleading for mercy, but Abigaille dismisses him.
In the next scene, on the banks of the Euphrates, the Jews remember their homeland in the famous chorus, "Va pensiero," and Zaccaria foresees the fall of Babylon.
PART FOUR: The Broken Idol: Alone in his palace, Nabucco hears the procession leading Fenena to her death. He has no legal power to stop it, so he makes one last desperate attempt: He prays to the God of Israel, in the aria "Dio di Giuda." The prayer is answered instantly, and Nabucco's insanity is cured. He summons a group of his warriors. When they realize their king is well again, he calls them to arms, and they head off to save Fenena.
The opera ends back at the hanging gardens, where Fenena and the Hebrews are about to be executed. Nabucco arrives just in time. He gives orders for the idol of Baal to be destroyed, and it immediately crumbles to dust. Nabucco then demands that the Jews be released, and that Babylon erect a temple to the Hebrew god. Finally, Abigaille is carried in. Knowing that Nabucco will condemn her, she has taken poison. Abigaille begs Fenena for forgiveness, and dies.
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