PROLOGUE
Scene One: The Huns are destroying the city of Aquileia, in northern Italy. King Attila's slave, Uldino, offers his master a captured band of warrior women as tribute. Their leader, Odabella, sings the praises of Italian women and their fierce patriotism in the aria "Allor che i forti corrono," and Attila is immediately smitten with her. He offers her anything she desires; when she asks for a sword, he gives her one that, unbeknownst to him, belonged to Odabella's family. In her cabaletta, Odabella vows to use this sword against him.
Attila then parleys with Ezio, the ambassador from Rome. In their duet, "Tardo per gli anni," Ezio asks Attila to help him overthrow the current Roman Emperor. Disgusted by such corruption, Attila vows, as scourge of God, to purge Rome of its decadence. He and Ezio are enemies once again.
Scene Two: A group of refugees from Aquileia arrives at an archipelago in a lagoon. Their leader, Foresto, laments losing his beloved Odabella to the Huns (aria: "Ella in poter del barbaro"), then, in a cabaletta, inspires the refugees with a vision of the great city they will build on these islands.
ACT I
Scene One: Foresto, in disguise, seeks out Odabella—now Attila's slave—at the camp of the Huns' army. They sing the duet "Si, quell'io son," discussing the jealousy and misunderstandings that threaten their relationship.
Scene Two: Attila, meanwhile, is roused from sleep by a nightmare. He tries to he brave before his men, but is terrified when his dream comes true: an impressive embassy from the Roman bishop Leone urges him to leave Rome untouched.
ACT II
Scene One: Ezio sings the aria "Dagli immortali vertici." Frustrated by the peace Attila has made with the Roman Emperor Valentinian, he agrees to join forces with Foresto and attack Attila.
Scene Two: At a feast, Foresto and the duplicitous Uldino attempt to poison Attila. Odabella, who wants to kill Attila herself, knocks the cup from the king's hands—then demands that, as reward for saving his life, Attila allow her to punish Foresto. Attila agrees and, furthermore, says he will marry her himself the next day.
ACT III
The next day, Ezio and Foresto lie in ambush near where Attila is to wed Odabella. In the aria "Che non avrebbe il misero," Foresto wonders why he still loves Odabella after so much treachery. Odabella rushes in, distraught, pursued by Attila. At the climax of the final quartet, Odabella slays the Hun with her family's sword.