Jerusalem. Inside the temple. The Levites and the people lament the sad destiny of the Jews, defeated by the king of Babylon Nabucco, who is now at the gates of the city. The chief priest Zaccaria encourages his followers. The Jews are holding an important hostage, the daughter of Nabucco, Fenena, whom Zaccaria hands over to Ismaele, nephew of the king of Jerusalem, for safekeeping. However, Ismaele promises Fenena her liberty, because some time ago in Babylon he had been kept hostage and had been liberated by Fenena, who is in love with him. The two are organising their flight when Abigaille, supposed daughter of Nabucco, arrives at the temple at the head of a large contingent of Babylonians. She too is in love with Ismaele, and threatens to tell Fenena’s father of her planned escape with a foreigner; in the end, Abigaille declares she will keep silent if Ismaele gives up Fenena. But he refuses to accept this blackmail. Nabucco, at the head of his army, crashes onto the scene, having decided to sack the city. In vain Zaccaria, brandishing a dagger over the head of Fenena, tries to stop him; Ismaele intervenes and hands Fenena over, safe and sound, to her father. 

ACT II. 

The wicked. At the court of Babylon. Abigaille has learned of a document that reveals her true identity as a slave: hence, the Babylonians are in error to think she is an heiress to the throne. Nabucco, in the heat of battle, has nominated Fenena regent of the city, making Abigaille hate her even more. The high priest of Belo, allied with Abigaille, tells her that Fenena is setting free all the Hebrew slaves. Abigaille seizes the opportunity and contemplates taking over Nabucco’s throne. Zaccaria, in the meantime, announces joyously to the people that Fenena, in love with Ismaele, has converted to the Hebrew faith. She is joined by Abdallo, an old officer of the king, who reveals Abigailles’s ambitions and advises her to flee to escape Abigaille’s ire. But there is no time. Abigaille arrives at the head of her magicians, the high Priest and a crowd of Babylonians. But unexpectedly, Nabucco also arrives, plants the crown firmly on his own head and curses the God of the Jews. Then he threatens to kill Zaccaria. Fenena reveals her conversion to him, but he forces her to kneel before him in adoration no longer as king, but as a god. The God of the Jews strikes him down with a lightening bolt. Nabucco, terrified, falls in agony, while Abigaille puts the coveted crown onto her own head. 

ACT III. 

The prophecy. The hanging gardens at the court of Babylon. Abigaille on the throne receives honours from all the authorities of the kingdom. Nabucco tries in vain to regain the throne, but is stopped by the guards. In the following dialogue between the two, Abigaille, taking advantage of Nabucco’s unstable mental condition, makes him put his royal seal on a document condemning the Jews to death. In a moment of lucidity, Nabucco realises that he has also condemned his beloved daughter Fenena and pleads for her salvation. But Abigaille tears up the document attesting to her state as a slave and declares herself the only daughter and heir to the throne. Then she orders the guards to imprison Nabucco. On the banks of the Euphrates the Hebrews invoke their faraway homeland and once again Zaccaria tries to console his people with a prophecy that encourages them to have faith.  

Act IV. 

The broken idol. From his prison Nabucco sees Fenena taken to her death among the other Jews. In desperation he turns to the God of the Hebrews, converting to the faith. When Abdallo and a handful of soldiers still faithful to the king see Nabucco return to his senses and his strength, they decide to revolt guided by the old king. In the hanging gardens a funeral march is playing: the Hebrews condemned to death are arriving. Zaccaria blesses Fenena, a martyr. But Nabucco crashes in, the idol Belo falls shattering to the ground and all the prisoners are freed. Nabucco once more sits on the throne. Abigaille, dying of self-inflicted poison, asks the pardon of Fenena and augurs her marriage with Ismaele. Zaccaria prophecies Nabucco’s dominion over all the peoples of the earth.