Thus saith the Lord, the God of Israel; Go and speak to Zedekiah king of Judah, and tell him, Thus saith the Lord; Behold, I will give this city into the hand of the king of Babylon, and he shall burn it with fire. Jeremiah 34:2

The Temple of Solomon. 587 BC. Seeking refuge in the Temple of Solomon, the Israelites are mustering their strength to resist the fearsome Nabucco (Nebuchadnezzar), king of Assyria, who has attacked and is desecrating the city. Amidst the chaos, Zaccaria, the Israelites’ spiritual leader, hurriedly seeks shelter from the warfare along with his sister, Anna. In an effort to negotiate an end to the siege, Zaccaria’s forces have kidnapped Nabucco’s daughter, Fenena, and now hold her hostage. He implores his people to be resolute (“Sperate, o figli”) in their defiance of the occupying armies. Ismaele, nephew of the king of Jerusalem and an Israelite military leader, breathlessly recounts the horrors of the street combat that is swiftly approaching their stronghold in the Temple. Zaccaria believes his hostage will prove a powerful negotiation tool with his enemy; he hands custody of Fenena over to Ismaele, as the Hebrews bravely face the turmoil of the battle outside the sanctuary of the Temple walls.

It is soon evident that Ismaele and Fenena are not strangers but rather former lovers, having met while Ismaele was in Babylon on a failed diplomatic mission. Their romantic reunion is interrupted when Abigaille, Fenena’s sister, infiltrates the temple. She scornfully turns on Ismaele as he appears to be a warrior weakened by love. She then privately tells reveals to him the true motive for her actions – she is desperately in love with him; implying that she has the power to call a cease-fire, if only he would return her affection (“Io t’amavo”). Unable to deny his love for Fenena, he offers to forfeit his life for his people. Hebrew refugees and wounded warriors are driven back into the Temple fearing for their lives as the Assyrian forces overpower them. As Nabucco raids their most sacred stronghold, Zaccaria condemns his arrogance and threatens to viciously murder Fenena in front of everyone, but Ismaele intervenes to stop the brutal execution. As the Jews turn on Ismaele for his treachery, Nabucco orders the temple sacked and burned and declares to his troops that it shall be a crime to show any mercy to the Hebrews.

Behold, the whirlwind of the Lord goeth forth with fury, a continuing whirlwind: it shall fall with pain upon the head of the wicked. Jeremiah 30:23

Several months later in Nabucco’s Palace in Babylon. Nabucco has left the palace to wage another war, leaving Fenena in charge as regent Queen in his absence, infuriating Abigaille who believes that she, as the eldest daughter, should have that right. Confirming her growing suspicion, she has found an official document proving that she is not Nabucco’s daughter but rather the progeny of slaves with no line of rightful succession to the throne. Angry and hurt at her father’s lies, she swears vengeance on Nabucco and Fenena while reflecting on the fact that her devotion to Ismaele could have changed everything for the better (“Anch’io dischiuso”) had he returned her affection. The corrupt High Priest of Baal interrupts her nostalgic memories to announce that Fenena, in a gesture of humanity and goodwill, has liberated the captive Israelites. To swiftly punish her treachery against the kingdom, the religious leaders have hastily arranged Abigaille’s ascension to the throne, deceiving the people by falsely announcing that Nabucco has been slain in battle. Abigaille feels she is vindicated and the fateful events have restored her to the honor she deserves (“Salgo già del trono aurato”)

A gallery in Nabucco’s Palace. Zaccaria, mindful of the menace and danger that his people are in, calls upon his faith to give him the resolve to continue fighting (“Tu sul labbro”). He believes that he may have a chance by converting Fenena, as she has shown compassion for his people’s plight. The Levites scold Ismaele for betraying his people, but Zaccaria reassures them that Ismaele has not betrayed them, as Fenena herself has now converted and has vowed to be faithful to the God of Israel. Abdallo, a Babylonian guard, rushes in with (false) reports of the king’s death, warning that Fenena is now in imminent danger.

A Hall in the Palace. The High Priest of Baal, followed by Abigaille and her supporters, enters to proclaim Abigaille ruler; her first act as Queen shall be to condemn the Hebrews to death. When Abigaille demands the royal crown that Nabucco has entrusted to his younger daughter, Fenena refuses to yield it. To the astonishment of all, Nabucco – very much alive, if mentally unstable – suddenly bursts forth, seizes the crown and places it on his own head. Everyone trembles before the irate tyrant (“S’appressan gl’istanti”). Sensing that his power is slipping away from him, he brazenly announces that he is not only their king, but also their God. As he tries to command everyone in his presence to prostrate themselves before him, his insanity becomes utterly apparent to all. Abigaille seizes the moment of personal and political turmoil to usurp the crown from the deranged King.

Therefore the wild beasts of the desert with the wild beasts of the islands shall dwell there, and the owls shall dwell therein: and it shall be no more inhabited for ever; neither shall it be dwelt in from generation to generation. Jeremiah 50:39

The Hanging Gardens of Babylon. Abigaille, in collusion with the religious leaders, exults in the praise of her subjects. In an effort to suppress any further insurrection, the High Priest of Baal exhorts her to decimate the Hebrews. Nabucco, descending further into madness, arrives to see his throne now occupied by his illegitimate daughter. Taking advantage of his senility, she convinces him to give his approval to a death decree against the Hebrews that she has drafted. Having doubts, he wavers, but her venomous taunts soon convince him. When he asks what has become of his beloved daughter Fenena, Abigaile says that she has converted to the Jewish faith and will therefore be executed with the others. Realizing that Abigaille is completely out of control, he searches in vain for the parchment which would reveal that she has no right to the throne. She produces it, and quickly destroys the evidence. He cries out for his guards to assist him, but they are no longer loyal to him. As he begs Abigaille to show clemency for Fenena’s soul, his guards, who have now defected from him, follow Abigaille’s orders and escort him away to prison.

The banks of the Euphrates River. Taking a moment of respite from their forced labor along the banks of the cool waters, the Hebrews recall with affection their homeland and the Jordan River from which they are exiled. Their plaintive cries nostalgically ascend “on golden wings” to the memory of their native soil (“Va, pensiero”). Zaccaria encourages them to not give up hope, but rather to use the power of their anger to break their chains of captivity and to strike their oppressors in brutal retaliation.

Declare ye among the nations, and publish, and set up a standard; publish,and conceal not: say, Babylon is taken, Bel is confounded, Merodach is broken in pieces; her idols are confounded, her images are broken in pieces. Jeremiah 50:2

A prison cell.. Wracked with guilt and suffering from a worsening mental disorder, Nabucco is unable to discern whether he is even awake or trapped in a nightmare. He believes that Fenena is being led away to face her fate to which he has doomed her. Losing the last shred of his faith, he implores the God of the Hebrews (“Dio di Giuda!”), praying for forgiveness and pledging to convert his people. Desperately trying to intervene on his daughter’s behalf, he realizes that he is indeed a prisoner and is powerless to help her. He believes that he is being saved by Abdallo and that his army is once again loyal to him. The execution of the death decree that he earlier consented to is now playing out before him. He hears Zaccaria hailing the beautiful Fenena as a martyr to the cause of the Israelites as she resigns herself to the impossibility of the situation (“Dischius’è il firmament”). Distraught, Nabucco rebukes Baal and commands that the idol of Baal be destroyed as a sign of his conversion. Nabucco’s senses once again failing him, he wonders if he sees Abigaille approaching. Appearing to have poisoned herself upon realizing the horrors that her hunger for power has brought upon her kingdom, Nabucco hears her confessing her crimes, hoping that it is not too late for Ismaele and Fenena to be reunited;slipping out of consciousness, she prays to the God of Israel for pardon (“Su me…morente”), as the voice of the Israelites collectively reaffirms that their God will always raise up those who are afflicted.