Premiere: La Fenice, Venice, 1823. Semiramide encourages her lover Assur to murder her husband, King Ninus. Her son, Ninius, disappears, believed dead, and Semiramide rules in her own right. Fifteen years later, as the opera opens, she is about to announce the name of her successor. Idreno and Assur are the leading candidates for the throne and the hand of Azema, but it turns out that Semiramide has taken a fancy to young Arsace, her victorious military leader who has been summoned back to Babylon. However Arsace himself is in love with Azema, and she loves him. Not only that, but although he has been raised in the distant state of Scythia, something only the high priest Oroe knows is that Arsace is actually Ninius, the son of Ninus and Semiramide, rescued and spirited away to safety after the coup. As the queen announces that Arsace is her successor, the ghost of her late husband appears from his tomb, demanding that Arsace punish the late king’s murderers. Assur and Semiramide resort to mutual recriminations, while Oroe tells Arsace of his true identity. All the principals gather in Ninus’ tomb, where Assur has decided to kill Arsace. Oroe forestalls him by ordering Arsace to kill Assur, but the queen interposes herself in the darkness and is killed instead. Assur is arrested, and Arsace, horrified at having killed his own mother, finally accepts the throne with great reluctance.