Aida, daughter of the King of Ethiopia, having fallen a prisoner into the hands of the Egyptians, is given as a slave by their king to his daughter Amneris who, captivated by the grace and beauty of the unknown maid, takes her into favor. Radames, a young captain of the king's guards, loved by Amneris, suspecting a rival in her slave, swears to be avenged. Meanwhile war is again declared between Egypt and Ethiopia and Radames, appointed leader of the army by the High Priest of Isis, is invested with the sacred arms and departs to fight the Ethiopians who, headed by their king, have invaded Egypt. Radames defeats them and returns victorious, followed by the prisoners, among whom is the king himself, disguised as an officer.