Act One – Jerusalem
The Babylonian army, lead by Nabucco (Nabucodonosor), the king of Babylon, is just about to take over Jerusalem.
The people of Jerusalem pray for salvation to their god Jehovah. The High Priest Zaccaria holds Fenena – the younger daughter of the Babylonian king Nabucco – hostage of the Jews, and he relies on her to secure peace between the two nations. Fenena has surrendered to the Jews to warn her beloved lsmaele – nephew of the Jewish king, of the danger. They have been in love for a long time, since the time lsmaele was an envoy of the Jews in Babylon.
The elder sister of Fenena – Abigaille is also in love with lsmaele and she leads a Babylonian army unit into the invasion. She would spare Fenena and the Jews if lsmaele gave up his love for her sister. But lsmaele refuses to do so and Abigailleswears to have her revenge.
The army of Babylon, lead by its king – Nabucco comes to the sound of a victory march. The High Priest of the Jews, Zaccaria, threatens to kill Fenena if his people are not spared. Ismaele stops him and for this he is cursed as a traitor by his Jewish brothers.
Act Two – The Unbeliever
Abigaille’s thirst for revenge is unquenchable. She has learned the truth about her descent – she was born the illegitimate daughter of Nabucco to a slave mother.
Abigaille accepts the proposal of the High Priest of Babylon to take by deceit the crown of Fenena, who in the absence of Nabucco temporarily holds the throne. The priests will help her spread the rumour that Nabucco is dead.
The Jews with their High Priest Zaccaria are secretly gathering to pray. Anna – Zaccaria’s sister reveals that Fenena has adopted their religion and Ismaele has actually saved a Jew.
Abdallo – the commander of the Babylonian king Nabucco, tells Fenena the news – Nabucco is dead, Abigaille and the priests have taken over the throne and her sister has sentenced her to death together with all the enslaved Jews. Fenena decides to fight and protect the Jews. Nabucco returns from the battlefield, victorious again. Intoxicated by his glory, he declares himself god on earth. But suddenly a lightning strikes him and he loses his mind.
Act Three – The Prophesy
Abbigaille has taken over the throne. She tricks Nabucco and with dimmed mind he signs the death sentence of Fenena and the Jews.
The enslaved Jews dream of their motherland. The prophetical words of Zaccaria about their future liberation reinforce their spirit.
Act Four – The Shattered Idol
Through nightmares and dimmed thoughts, Nabucco finally realizes the truth of the death that is upon Fenena.
He sends a plea for help to the god. A miracle happens – Nabucco comes to his senses. With his loyal Abdallo and a unit of his army he hurries to help his daughter. Nabbuco frees the slaves and reunites Fenena and lsmaele. A joint prayer unites the two nations – Jews and Babylonians. A contrite Abigail joins her supplicants in the last moments of her life.