The opera opens in the Temple of Soloman in Jerusalem.  The Babylonian army is attacking the city.  The High Priest, Zaccaria, brings a message of hope....and a hostage: Fenana, who is the daughter of the Babylonian's leader Nabucco.  The crowd disperses, leaving the young soldier Ismael with Fenana.  They are in love, and Ismael wants to save her, but Abigaille bursts in on them.  She is Nabucco;s other daughter, who also loves Ismael.  Abigaille offers Ismael his freedom, but he rejects the offer and her.  Suddenly Hebrews burst in with the news that the Babylonians have entered the city.  Nabucco himself enters threatening destruction, but Zaccaria intends to slay Fenena.  Unexpectedly, Ismael save Fenena from Zacharia.  Nabucco unleashes his fury:  joined by Abigaille, he incites the Babylonians to sack the city.  The Hebrews curse Ismael as a traitor.

Abigaille is in possession of a document proving her to be the daughter of slaves.  Nabucco has left Babylon and appointed Fenena regent in his stead.  This infuriates Abigaille, who lusts after power.  The High Priest of Bel arrives with the news that Fenena intends to free the Hebrew slaves.  He encourages Abigailleto seize power from Fenena.  He has also spreat the rumour amont the Babylonians that Nabuccvo has died in battle.  This, he says, will pave the way for Abigaille's coup.  Abigaille is thrilled at the prospect of taking the throne.

Zaccaria enters a chamber within the palace in Babylon.  With him are the Levite and Hebrew Soldier, who bear the Tables of the Law.  Zaccaria is preparing to accept Fenena into the Hebrew faith.  He leaves but the Levite encournters Ismael, who has come to plead forgiveness.  Anna, Zaccasria and others enter to announce that Ismael has been forgiven due to Fenena's conversion.  Abdallo enters to announce Nabucco;s death and Abigaille's ascension.  Abigaille enters and tries to wrest the crown from Fenena, but finds herself confronted by none other than Nabucco.  He takes the crown and curses the Babylonians who have betrayed him, declaring "I am no longer a king, but GOD!"  He is struck by lightning and falls to the ground, insane.

Abigaille is in the throne room; she has taken power from the ailing Nabucco.  The High Priest of Bel urges her to sign the document condemning Fenena and the Hebrew Captives to death.  Nabucco arrives semi-delirious.  She tricks him into signing the document himself.  When he realizes what has happened, he challenges Abigaille and demands she yield to her master.  She produces the slavery document and tears it up in contempt.  Trumpets sound, announcing the impending death of Fenena and the Hebrews.  Nabucco pleads with Abigaille, but in vain.

The Hebrews have gathered on the banks of the Euphrates River.  We hear the famous "Va pensiero..."  the Chorus of the Hebrew Slaves.  Zaccaria comes to reasure them with the prophecy of freedom.

Nabucco is seen in his prison cell, cringing from nightmasres.  Hearing the crowd as they prepare to sacrifice Fenena, ho looks out and sees the procession, at first assuming them to be honouring Fenena.  He is proud to witness her walking along amidst the soldiers, but suddently notices that she is in chains.  The shock breaks him out of his delirium.  He recognizes that he's a prisoner and falls to his knees, begging the God of the Hebrews to forgive him.  Nobucco is miraculously restored to sanity just as Abdallo bursts in with his sword.  Nabucco rushes off to save Fenena.

Assembled in the Temple of Bel, the Babylonians are about to sacrifice Fenena.  Zaccaria blesses her, and she sings a hymn, accepting her impending death.  Nabucco bursts in and calls for theoffending statue of Bel to fall to the ground.  The crowd is awed by the miracle.  Nabucco acknowledges the power of the God of Israel, and calls on all the people to kneel in prayer.  Abigaille enters slowly and painfully.  She had taken poison.  She bege the forgiveness of Fenena, and dies.