The scene of the opera is laid in Babylon, and the story briefly told is as follows: Ninus, the King of Babylon, has been murdered by his Queen, Semiramis, aided by Assur, a prince enamoured of her and aspiring to the throne. One of the Queen’s warriors, Arsaces, supposed to be of Scythian origin, but in reality her own son, returns from a foreign expedition and is loaded with honors for the victory he has won. Semiramis, ignorant of his parentage, has a secret passion for him, he in the mean time being devoted to Azema, one of the princesses royal. As all gather together in the temple to swear allegiance to the Queen, the gates of Ninus’s tomb suddenly open, and his ghost appears and announces that Arsaces will be the successor to the crown. At midnight Semiramis, Assur, and Arsaces meet at the tomb, and by mistake Assur stabs her instead of Arsaces, who in turn kills Assur, and, all obstacles being removed, is united to Azema and ascends the throne.
An introductory chorus of Babylonians and a terzetto by Idreno, Assur, and Oroe open the opera and lead up to the first appearance of Semiramis, which is followed by a very dramatic quartet ("Di tanti regi"). In the fourth scene Arsaces has a very brilliant aria ("O! come da quel di"), which also did service in one or two of Rossini’s other operas, and is followed by a very animated duet ("Bella imago degli dei”) between himself and Assur. The eighth scene is introduced by a graceful female chorus which leads to Semiramis’s brilliant and well-known aria, “Bel raggio.” In the tenth scene occurs an elegant duet ("Serbami agnor si fido"), followed in the next scene by a stately priests’ march and chorus ("Ergi omai la fronte altera"), set to ecclesiastical harmony and accompanied by full military band as well as orchestra, this being the first instance where a military band was used in Italian opera. It leads to the finale, where Semiramis on her throne announces to her people her choice for their future king. The oath of allegiance follows in an impressive quartet with chorus ("Giuro al numi"), and a defiant aria by the Queen leads to the sudden appearance of the ghost of Ninus, accompanied by characteristic music repeated in quintet with chorus. As the ghost speaks, the statue scene in Don Giovanni is inevitably recalled, especially in some phrases which are literally copied.