The young king Xerxes pursues Romilda, determined to make her love him. But Romilda loves Xerxes' brother, Arsamene, who loves her in return. Xerxes banishes Arsamene to be rid of his rival. Meanwhile, Romilda's sister, Atalanta, has her eye on Arsamene and makes overtures to him, intending to make him her own.
Arsamene sends a letter to Romilda, via his servant Elviro (dis-guised as a flower girl), but Atalanta intercepts it. She tells Xerxes that Arsamene loves her and wrote her the note. So, she says, Xerxes may feel free to pursue Romilda. Atalanta is temporarily successful in driving a wedge between Romilda and Arsamene. The separated lovers each suffer a crisis of faith, and they argue bitterly.
In the meantime Xerxes is relent-less and even cruel in his pursuit of Romilda. He tells the girls' father, Ariodate, with an air of mystery, that someone of Xerxes' family and equal to Xerxes will come to claim Romilda. Ultimately, Ariodate decides Xerxes means Arsamene, since he is, of course, of Xerxes' family, and equal to him. So the lovers are finally reunited—indeed, married—though this is hardly what Xerxes intended.
Amastre, the fiancée Xerxes has forsaken for Romilda, spends the opera disguised as a man. She comes to town to find Xerxes, from whom she has heard nothing. She watches him unobserved, learns of his treach-ery, struggles with conflicting feelings of pain and love, and finally writes him a farewell note and plans to leave. Ultimately, when all parties confront one another, Amastre makes herself known to Xerxes, who is terribly ashamed. He asks her, with great gentleness, to stay. He has learned a lot from his impassioned pursuit of Romilda, though not what he expected, and all his longing for love is now transferred to someone who loves' him. Atalanta is left without a lover, severely chastened by those she has deceived and much, much wiser.