It is 587 BC and the Babylonian army storms Jerusalem and threatens the gates of the temple. The Israelites lament their fate, but the prophet Zaccaria (bass), High Priest of the Hebrews, tells them he has taken a hostage: Fenena, daughter of Nabucco (baritone), the Babylonian king [Nebuchadnezzar in the Bible], and God will be on their side. The Israelites follow Zaccaria into battle. Meanwhile, Ismaele (tenor) and Fenena (soprano) fell in love while Ismaele was imprisoned in Babylon, and Fenena has helped him escape to Israel. Their song is interrupted by Abigaille (soprano), who arrives leading a band of disguised Assyrian warriors. A slave, presumed to be the first daughter of Nebuchadnezzar, she had also fallen in love with Ismaele during his captivity and now taunts him with her victory. The Israelites panic as they face defeat. Nabucco arrives on horseback, but Zaccaria threatens to kill Fenena if Nabucco profanes the temple. Ismaele, fearful for Fenena, disarms Zaccaria.