The action of the first part takes place in Jerusalem and other parts in Babylon during the reign of Nebuchadnezzar II (605-562 BC), who in 586 BC destroyed the Temple of Solomon in Jerusalem.
Part I JERUSALEM
The interior of the Temple of Solomon
The Babylonians (also referred to as Assyrians), led by King Nabucco (Nebuchadnezzar) are about to attack Jerusalem. The High Priest of the Hebrews, Zaccaria (Zechariah) hopes that Nabucco's daughter Fenena, whom he holds hostage, will help secure peace. Fenena is placed under the care of Nabucco's nephew Ismaele (Ishmael) while Zaccaria and the Hebrews go to defend the city.
Ismaele and Fenena are secretly in love. Their flight together is interrupted by a band of Babylonian soldiers disguised as Hebrews. They are led by Nabucco's eldest daughter Abigaille, also in love with Ismaele. After confronting the two lovers, she confesses her own feelings and offers to save the Hebrews for his love. Ismaele refuses.
The Hebrews retreat back to the Temple. Nabucco and his advancing army are stopped by Zaccaria at the Temple threshold. Nabucco apperas to yield as Zaccaria threatens to kill Fenena.
Ismaele suddenly wrests the dagger from Zaccaria's grasp. With Fenena now safe, Nabucco orders the Temple burned. Abigaille denounces her love and vows to annihilate the Hebrews. Zaccaria curses Ismaele for freeing Fenena and betraying his people.
Part II THE WICKED MAN OR THE UNBELIEVER
An apartment in the Royal Palace of Babylon
Abigaille discovers she is not the King's daughter but a slave. Still ambitious for power, she rages against Nabucco for appointing Fenena as regent while she herself remains in Babylon. She plans revenge against Fenena - her rival for love and the throne. The High Priest of Baal reports to Abigaille that Fenena is freeing the Hebrews. Desperate, Baal spreads a rumor of Nabucco's falli n battle. Abigaille exalts in the thought of ascending the throne.
Zaccaria intends to convert Fenena to the Jewish faith. Ismaele, Zaccaria's sister Anna and Fenena gather in the hall. The King's death and Abigaille's rise to power is reported. The latter enters with High Priest of Baal to snatch the crown from Fenena. Suddenly, Nabucco apperas and takes the crown to himself. After commanding the Hebrews to succumb and hail the „new God", a thunderbolt rips the crown from his head. Nabucco apperas to go mad. The crown is retrieved by Abigaille.
Part III THE PROPHECY
The Hanging Gardens of Babylon
Abigaille now assumes power. The High Priest of Baal asks for death for Fenena and the Hebrews. Disoriented and unable to yield to Abigaille's taunts, Nabucco signs the death warrant. He then attempts to save Fenena by producing the document proving Abigaille's lowly birth. She tears it apart and places Nabucco under arrest. His pleas to save Fenena are in vain.
The enslaved Hebrews long for their lost homeland. Zaccaria prophesies and end to their suffering and the destruction of Babylon.
Part IV THE BROKEN IDOL
An apartment in the Palace
Stirred awake, Nabucco sees Fenena being led to her execution. He begs forgiveness to the Hebrew God and promises to rebuild the Temple. His mind clears. He rushes off to save Fenena.
The Hanging Gardens of Babylon
Nabucco and his soldiers arrive to save Fenena and the Hebrews. He orders the destruction of the image of Baal. It falls and shatters. Nabucco bids the Hebrews to return to their country and prasies the Lord. Abigaille has taken poison and nears death. She begs Fenena forgiveness and, upon seeing Ismaele, urges Nabucco to protect the two lovers. She dies invoking the God of the Hebrews.