The action takes place first in Jerusalem, in Solomon’s temple, and later Babylon, during the reign of Nebuchadnezzar (605-562 BC).
ACT 1
Solomon’s temple
The Hebrews and Levites are praying that their temple may be spared from the Babylonians’ fury. Zaccaria, the High Priest, arrives and reveals that he is holding Nabucco’s daughter, Fenena, hostage. He hopes that the young girl may be the means of negotiating peace. Ismaele, nephew of the King of Jerusalem and in love with Fenena, arrives with the news that Babylonian soldiers have already broken in.
Zaccaria asks Ismaele to keep watch over Fenena while he is leading the people in an attempt to defend the city and temple. Ismaele and Fenena exchange tender words. They have long been lovers, since Ismaele was sent to Babylon as an ambassador. While there, he was cast into prison but managed to escape with Fenena’s help. Fenena’s sister Abigaille is also in love with him but did not want him to escape. Ismaele now feels indebted to Fenena and is determined to run away with her.
If Ismaele returns her love, she will call off the hostilities and leave the Hebrews to live in peace.
In the meantime a group of Babylonian soldiers disguised as Hebrews reach the temple. They are led by Abigaille, presumed to be Nabucco’s firstborn. Indignant at first, she pours venom on the two lovers before proposing a pact: if Ismaele returns her love, she will call off the hostilities and leave the Hebrews to live in peace. Ismaele refuses. The defenders of Jerusalem enter the temple, followed by Nabucco on horseback. Zaccaria threatens to kill Fenena if Nabucco and his people violate the temple. Nabucco hesitates and is inclined to withdraw from the temple, but soon returns and curses the Hebrews’ god. Zaccaria makes to kill Fenena but is prevented by Ismaele. Nabucco, embracing his daughter again, orders his soldiers to loot and set fire to the temple. The disappointed Abigaille unleashes her hatred of the Hebrews. Zaccaria curses Ismaele, who, for the love of a Babylonian woman, has betrayed his people and caused the destruction of the temple.
ACT II
The royal palace in Babylon
Abigaille enters. She is distraught on finding a paper she believes reveals her true ancestry. She is not Nabucco’s firstborn but the daughter of a slave. Though uneasy and anxious, she decides not to renounce the benefits due to a king’s daughter. She is angry with her father for not trusting her to lead the army and ordering her to return to Babylon. What is more, he has entrusted the regency to Fenena until the end of the campaign. Abigaille is blinded by her hatred for her sister, who has stolen both her love and her power.
The High Priest of Baal indignantly announces that Fenena is intending to release the Hebrews. Abigaille feels that the time has come to act. She must prevent Fenena from carrying out her intention. The High Priest agrees with her that they must put about a false rumour that Nabucco has died in battle. Abigaille can then seize the crown.
She is not Nabucco’s firstborn but the daughter of a slave.
In another wing of the palace Zaccaria and his people are going to Fenena’s apartments to make her convert to the Jewish faith. Ismaele, alone in a room with the Levites, is deeply dejected; he is seen by all as a traitor. Zaccaria returns with Fenena and her sister Anna and orders the Levites to stop insulting Ismaele. He is no traitor: he has saved a converted woman from death. Abdallo rushes in with the news that Nabucco is dead and Abigaille has ascended to the throne.
Abigaille arrives with a handful of soldiers to wrest the crown from Fenena just as Nabucco enters the scene, claiming the crown and declaring the fall of all the gods. From now on, he is not only King but also God. No sooner has he uttered these words than he is struck by a thunderbolt. Taking advantage of his bewilderment, Abigaille snatches the crown and places it on her head.
ACT III
The hanging gardens of Babylon
Abigaille, having deposed her father, is a fierce ruler. The High Priest of Baal brings her a death sentence to sign on all the Hebrews and her sister, Fenena. Suddenly Nabucco appears, shabbily dressed and seeming to be out of his mind. Alone with her father, Abigaille asks for the death sentence to be signed. He hesitates, but when she makes fun of him, he signs it out of spite. Not until the sentence is in the hands of the guards does he realise he has signed the death sentence for Fenena, too. Abigaille is exultant. Nabucco, aghast, threatens to reveal her true identity. He looks for the birth certificate but, laughing, Abigaille tears it into shreds before his eyes. She can now do whatever she wants. She calls for the guards and has the old king imprisoned. Nabucco, in despair, promises Abigaille that he will abdicate in her favour if she will pardon Fenena. This she scornfully refuses to do.
Suddenly Nabucco appears, shabbily dressed and seeming to be out of his mind.
On the banks of the Euphrates the Hebrews, in chains, pause in their labours and sing for their lost homeland. Zaccaria urges his people to take heart, predicting that their trials will soon end and that Babylon will be destroyed.
ACT IV
Scene 1
The royal palace
Nabucco wakes from a nightmare. Hearing shouts, he calls for arms and a horse to lead an attack. He soon discovers that the voices are those of the crowd accompanying Fenena on her way to her execution. With his daughter on the scaffold and himself a prisoner in his own palace, he feels utterly powerless. He prays to the God of Israel, swearing that he will have the temple in Jerusalem rebuilt if Fenena is saved. At the palace door he meets Abdallo and other soldiers still faithful to him. He is now ready to attack, to punish the traitors and rescue his daughter.
He soon discovers that the voices are those of the crowd accompanying Fenena on her way to her execution.
Scene 2
The hanging gardens of Babylon
Fenena and the condemned Hebrews are brought in to the accompaniment of a funeral march. Zaccaria consoles Fenena as she goes forward to her martyr’s death. All of a sudden voices are heard praising Nabucco. The High Priest of Baal urges his men to hurry with the execution, but time is running out. Nabucco and his men break in and stop the slaughter. He then orders the image of the God to be overthrown, but it crashes to the ground before anyone touches it.
The Hebrews are free and Nabucco exhorts his people to bow before the mighty God. Abigaille is nowhere to be seen. Defeated in every way, she takes poison and, with two of her followers, goes to the spot where the Hebrews were to have been executed. Before dying, she begs her sister’s forgiveness and asks Nabucco to protect the two lovers, Ismaele and Fenena, and permit their marriage. She dies invoking the God of the Hebrews.
