Xerxes, the young king, ripe for love, directs his amorous attentions to a favorite tree. His brother, Arsamene, sneaks past with his servant Elviro, en route to woo his secret love, Romilda.

Romilda sings a song from her balcony, mocking Xerxes’ passion for the tree, galvanizing both brothers, who stumble upon each other. Arsamene pretends ignorance of the singer. Xerxes, a headstrong and hot-tempered young man, decides to pursue Romilda and, sensing Arsamene’s discomfort, declares that his brother must tell Romilda the good news, then exits. Arsamene wavers between confidence in Romilda’s loyalty to him and fear of his brother’s willfulness. Romilda reassures him.

Enter Atalanta, Romilda’s sister, also ripe for love. She directs her amorous attentions to Arsamene. This annoys Romilda. Xerxes returns to claim Romilda as his bride; he is blunt and charmless. He goads Arsamene, realizes his brother is his rival, and banishes him. Suddenly young love is no longer innocent; the stakes rise. Xerxes begins his relentless and sometimes cruel courtship of Romilda, who is distraught and turns a deaf ear to him. She quiets her fear with a solitary vow of constancy.

Enter Amastre, a foreign princess betrothed to Xerxes. She has heard nothing from her fiancé and has come, disguised as a man, to find out why. Enter Ariodate, father of Romilda and Atalanta, soon joined by Xerxes. Amastre hides. Xerxes tells Ariodate that Romilda will soon be wed into the royal family, to someone “equal to Xerxes.” Ariodate expresses delight and gratitude.

Xerxes, believing he is alone, voices his desire for Romilda. Amastre, outraged, almost reveals her identity but cannot bring herself to confront Xerxes. She listens sadly as he daydreams ecstatically about love. When Xerxes wanders off she also overhears a conversation between Arsamene and Elviro, who have crept back with a letter for Romilda. Arsamene orders Elviro to find a way to remain near the house until an opportunity arises to deliver the letter. Amastre, having observed Xerxes’ passion and Arsamene’s resultant misery, angrily determines to revenge herself on Xerxes.

Atalanta preys on Romilda (who is credulous and touchy since Arsamene left), insinuating that Arsamene already loves someone new. Romilda sees through her sister’s ploy and vigorously warns her not to pursue Arsamene. But Atalanta is perversely determined to drive a wedge between the lovers. She considers other strategies and takes delight in reviewing her various talents.

Elviro, dressed as a flower seller, Arsamene’s letter in hand, haunts the road outside Romilda’s house awaiting his moment. Amastre engages him in conversation and learns that Romilda does not love Xerxes though Xerxes remains obsessed with Romilda. Amastre leaves in a huff.

Atalanta is Elviro’s next customer; when she recognizes him she takes the letter and promises to deliver it to Romilda directly. She tells a lie: Romilda is at that moment writing a love letter to the king. Appalled, Elviro leaves in a huff.

The letter now passes from Atalanta to Xerxes: she says it is from Arsamene to her (another lie). Xerxes, delighted to hear that Arsamene does not love Romilda, agrees to marry Atalanta to his brother at his earliest convenience. Atalanta tells yet another lie: Arsamene will persist in declaring that he loves Romilda, but it isn’t true – “he pretends he loves her so she won’t suspect that we are …”

Xerxes now shows Romilda Arsamene’s letter, and Romilda believes herself betrayed. To Xerxes she says she will always love Arsamene; to herself she says she will not. She suffers terrible jealousy and anger at Arsamene’s betrayal.

And Amastre suffers terribly at Xerxes’ betrayal. Elviro prevents her from harming herself, and she decides finally to confront Xerxes.

Elviro reports Atalanta’s lie to Arsamene: Romilda loves the king. Master laments, servant comforts.

Xerxes embraces Arsamene with the news that they’ll both be wed to the women they love, and they are friends again … until Arsamene discovers that Xerxes still wants Romilda. Xerxes finds Arsamene’s persistent declarations of love for Romilda hilarious: it’s just as Atalanta predicted! Eventually Arsamene convinces Xerxes that he does not love Atalanta, and Xerxes whirls furiously on Atalanta and tells her to quit her conniving and find someone else to love. But Atalanta, suddenly vulnerable, feels she may have come to love Arsamene truly.

Xerxes ponders the uncertainty of love.

Elviro finds refuge from his sadness in drink and falls asleep.

Xerxes and Amastre pine for their loves. Xerxes overhears Amastre curse the king, and the two lock swords, but Romilda forces them to disengage and admonishes them both: “The man who allows his confusion to rule him … no lover is he.”

Romilda and Arsamene argue heatedly then confront Atalanta with her lies, scorning her feeble excuses. Atalanta is shamed and hurt and stomps away.

Arsamene hides as Xerxes returns to pressure Romilda. He says he can and will, as king, force her to wed him. She says she will have to obey her king, and he leaves triumphant. Arsamene bursts out of hiding rebuking Romilda – how could she say what she said? They argue again. Their problems seem insurmountable. They separate, possibly forever.

Xerxes tells Ariodate that the promised royal wedding will occur within the hour, when someone of his family and equal to him appears to claim Romilda. Ariodate assumes Xerxes means Arsamene.

Romilda asks Amastre to carry a message to Arsamene – that she loves him only and that they must elope to save their love. Amastre, in return, asks Romilda to deliver a letter to Xerxes for her. In deciding to confront Xerxes, Amastre has recovered her dignity; she realizes she will be a victim only if she allows herself to be a victim. She goes off to find Arsamene. He enters from another direction to find Romilda with suitcases in her hands, but before she can explain they are again fighting bitterly.

Ariodate finds them and, before they can explain that they are fighting, marries them, “as per the king’s decree.” They are dumbfounded. Everyone is delighted except Xerxes, who returns in his wedding clothes, full of anticipatory glee, just after the ceremony. Ariodate must break the news to him. In addition, Amastre’s letter to Xerxes arrives. Xerxes flies into a rage.

He is now forced to come to terms with things as they really are, not as he would like them to be. He must acknowledge that Romilda and Arsamene are irrevocably married and meant for each other, and when Amastre confronts him he is terribly ashamed and 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.