This is Rossini's first opera, it was commissioned by the tenor Domenico Mombelli, whose wife prepared the libretto. The lyrical love duet (questo cor ti giura amore), which Rossini reused in several other operas, became very popular.

The story of rival dynasties, betrayals, and disguises takes place at the Persian Court.

Act 1

A young man known as Siveno (alto), Ignorant of his true lineage, lives at the court of Polibio (bass), king of the Parthians. He loves, and is loved by, Lisinga (soprano), the daughter of the king. Siveno arrived at the court of the Parthians when he was a child, brought by a minister of Demetrio, king of Syria, in order to escape a violent insurrection. Demetrio (tenor), disguised as an ambassador under the name of Eumene, arrives at Polibio's court to take back Siveno, but Polibio rejects his demands; on the contrary he decides to let Siveno marry Lisinga thus making him a successor to his throne. Eumene/Demetrio decides to abduct Siveno, but when he reaches the newlyweds' room he finds only Lisinga and takes her as a hostage.

Act 2

Siveno and Polibio go to Eumene's camp to recover Lisinga; but Eumene threatens to kill her if he cannot obtain Siveno. Polibio, on the other hand, threatens to kill Siveno if Lisinga is not freed immediately. While the newlyweds declare themselves ready to die for each other, Eumene notices a medal hanging from Siveno's neck and 'recognising his son, expects Siveno to return with him to Syria. The conflict between the two kings becomes even more dramatic and the bride and groom are separated by force. To regain her husband Lisinga plans to kill Eumene, but the latter eventually reveals himself as the King of Syria and Siveno's true father, and gives up on his demands for the sake of the happiness of the newlyweds, who can finally be reunited.