Semiramide has ruled for decades, after conspiring with her lover Assur to murder her husband King Nino. Now she is due to elect her successor. Assur assumes the crown is his, but is furious when Semiramide leaves it to the young officer Arsace, whom she loves. Only the high priest Oroe knows that Arsace is Semiramide and Nino’s son, lost since his father’s murder.
Arsace learns his true identity from Oroe. He forgives his mother and goes to kill Assur. The battle takes place in darkness. Semiramide, struck by a wayward blow, dies. Assur is arrested and a devastated Arsace is declared king.