Semiramide, with Assur, has secured the murder of her husband, King Nino. Her son, however, has escaped death and is now, as Arsace, a successful commander, his identity unknown to his mother. He is called back to Babylon, is in love with Azema and unwilling to support Assur in the latter's bid for the throne. Semiramide falls in love with him and declares him king and her consort, while Azema will marry Idreno. King Nino's ghost warns of crimes to be expiated and the high priest Oroe tells Arsace of the crime committed by his mother and Assur. Arsace, in the tomb of his father, meets King Nino's murderers, and seeking to strike Assur, kills Semiramis. He is finally declared King.