Queen Semiramide is haunted by the ghosts of her past. Together with her lover Assur, she once murdered her husband King Nino; a deed which ever since has weighed heavily upon her. With her marriage to Arsace, she hopes her soul will at last find solace. Her love, however, is misplaced. Arsace not only loves another, he is also, as is later revealed, the son Semiramide and Nino believed to be dead. He is faced with a decision: should he avenge the death of his father – and thus become his mother's killer?