Thamos has succeeded his father, Ramesses, as king of Egypt, but Ramesses had usurped the throne from the rightful king, Menes, who is now disguised as the high priest, Sethos. Thamos loves Sais, a priestess, but she is really Menes' daughter Tharsis, for whom the high priestess Mirza is plotting marriage to Pheron, a treacherous general.