The king of Macedonia, Alessandro, has overthrown Stratone, the tyrant of Sidon, but aims to find the rightful king.
In a meadow. The city of Sidon can be seen from a distance.
Elisa is with her lover Aminta, the shepherd. She assures him that the war between King Alessandro and Stratone, the tyrant of Sidon, will not affect their love for each other. Having deposed Stratone, Alessandro searches for the rightful heir to Sidon. He thinks that Aminta is the rightful heir. He comes to Aminta in disguise and offers him to take him to Alessandro. Aminta wants to remain a shepherd. Meantime, Agenore encounters his beloved Tamiri, daughter of Stratone. Tamiri is comforted to learn that Agenore still loves her.
Elisa gets permission from her father to marry Aminta. Agenore tells Aminta that he is the rightful heir to the throne and that his father was driven out by Stratone when he was a baby. Aminta promises to return to Elisa after claming his throne. Aminta loves Elisa but Alessandro suggests that when Aminta is hailed king, royal duties take precedence over love. Alessandro suggests that Tamiri marry Aminta in order to ascend her father's throne. Aminta disagrees.
Elisa is prevented by Agenore from seeing Aminta. He also discourages Aminta from pursuing her. Alessandro tells Aminta to dress like a king so he can be presented to his subjects. He also decides that Tamiri marry Aminta. Aminta is distraught. Agenore is upset. He breaks the news to Elisa. Tamiri does not want to marry Aminta. Agenore, too, is tormented by the planned marriage. Tamiri tells Alessandro that she and Agenore are in love. The women threw themselves in Alessandro's mercy. Elisa begs him to give her back Aminta who declares his love for Elisa ("L'amerò, sarò costante"). Realizing the potential injustice he was about to inflict, Alessandro tells Aminta to marry Elisa and Tamiri to marry Agenore. Aminta is crowned king of Sidon.