At the opera’s outset, Nabucco (in English, Nebuchadnezzar) and his Babylonian army are at the gates of Jerusalem, the defeat of the Jews imminent. Nabucco’s younger daughter, Fenena, is being held hostage by the Jews; but she has fallen in love with her jailer, Ismaele. The couple is preparing to escape when Fenena’s older sister Abigaille learns of their plans and threatens to expose them unless Ismaele becomes her lover.
Abigaille learns that she was adopted by Nabucco, and that he has named as regent Fenena, who converts to Judaism to placate the conquered people. Nabucco, declaring himself a god, is stunned, though not killed, by a lightning strike, whereupon Abigaille seizes the crown, and issues a death decree for all the Jews, including her recently converted sister. On regaining his senses, Nabucco sees Fenena being led as a sacrifice to the altar of the god Baal. Nabucco prays to the god of the Jews, leads a revolt against Abigaille, frees the Jews and promises to build them a new and bigger temple. Abigaille takes poison, seeks forgiveness and dies; the high priest Zaccaria proclaims Nabucco the king of the world.
The opera closes with the multitude rejoicing, including Fenena and Ismaele who are now free to marry.