The action takes place in 587. Ave. Cr. In Jerusalem, later in Babylon. Jerusalem. Solomon's Temple. Judeans gather there, surrounded by Nabucco's army. Among them is Fenena, the daughter of the Babylonian king. Abigail, the daughter of Nabuc, fell in love with Ishmael, the nephew of Judean king Zedekiah, while he was in Babylon. The outcast Abigail imprisons Ishmael, but Fenena rescues him and secretly arrives with him in Jerusalem. Meanwhile, the Babylonians are already storming the temple, including the vengeful Abigail. She promises Ishmael and his people freedom if the latter renounces Fenena for her. High priest Zacharias threatens to kill Fenen if the temple is destroyed, but Ishmael holds his hand. Nabucco's soldiers occupy the temple.
In the palace of the king of Babylon, Abigail finds a writing, testifying that she is Nabuc's illegitimate daughter, born to a slave. This makes her hatred for Fenena even stronger. After her father went to war, Abigail, using deceit and falsely spreading the news about Nabucco's death, wants to take the royal crown from Fenena by force. Nabuk suddenly returns and declares himself a god. Zechariah threatens him with God's punishment. At that moment, thunder strikes and tears the crown from the king's head. Nabuc goes crazy.
Wearing a crown, Abigail reigns. She prepares a death sentence for the Jews and Fenena. Nabuc, having lost his common sense, signs it. After regaining consciousness, he begs Abigail to spare his honor and Fenena's life, but Abigail's revenge is unrepentant.
On the banks of the Euphrates River, Jews remember their homeland and ask God to give them courage. The priest Zechariah prophesies the destruction of Babylon.
Nabucco regains consciousness, his mind brightens. As if in a dream, he hears shouts announcing Fenena's death. Overcome with grief, the king falls to his knees and prays to the God of the Jews, asking for forgiveness and swearing to worship him. With his loyal soldiers, he rushes to rescue Fenena and the other doomed. Nabucco overthrows the idol of Bel and praises the God of Israel. Desperate, Abigail drinks a cup of poison.
