Scene 1: A summer house on one side of a garden
Resting under a plane tree, Xerxes expresses his gratitude to the tree for its shade. Arsamenes is looking for his beloved Romilda who is in the summer house, deriding Xerxes for his vegetable enthusiasm. Hearing her voice, Xerxes is smitten with love for her and orders Arsamenes to tell her. When Arsamenes evades the task, Xerxes decides to tell her himself.
Although he swears that she will be his, Arsamenes wonders whether Romilda may be influenced by Xerxes' passion. She assures him that she will not be swayed away from her love for him, while her sister, Atalanta, who also loves Arsamenes, hopes that Romilda will incline to Xerxes. When Romilda tells Xerxes she does not aspire to be his queen, he blames Arsamenes and banishes him. He continues to press Romilda, who continues to reject him.
Scene 2: A courtyard
Disguised as a man, Amastre, to whom Xerxes is betrothed, comes to check up on him. She watches as Ariodates, Xerxes' general and father of Romilda and Atalanta, reports on his success in battle. To reward him, Xerxes promises that his daughter Romilda will have a royal husband. Ariodates leaves with his soldiers and Amastre listens as Xerxes soliloquises about his love, at first believing herself to be its object, but soon learning that he loves another.
She gives a hasty explanation for her exclamation of anger, Xerxes sends her away and continues to brood on his passion for Romilda. Arsamenes gives his servant Elviro a letter for Romilda, for whom he continues to pine.
Amastre broods on revenge for her slighted love. Atalanta tries to persuade Romilda to accept Xerxes, but Romilda sees through her pretence of disinterest and rejects the idea. Atalanta continues to hope.
Scene 1: A city square
The disguised Amastre learns from Elviro, who is pretending to be a flower-seller, that Xerxes wishes to marry Romilda, who loves his brother Arsamenes.
Elviro is recognised by Atalanta, who persuades him to give her Arsamenes' letter to Romilda, promising to deliver it, but telling him that Romilda has no thought of Arsamenes, intending to accept Xerxes. Xerxes discovers Atalanta reading Arsamenes' letter to Romilda. She lets him read it, claiming that she was the intended recipient. She says that while Romilda loves Arsamenes, he is only pretending to love her, to disguise his passion for herself.
Xerxes confronts Romilda with the letter, telling her it was written to Atalanta, but she still resists him and clings to Arsamenes, even if faithless, but when Xerxes has left, she gives vent to jealousy. Elviro prevents Amastre from killing herself. He tells Arsamenes that he gave the letter to Atalanta, who has convinced him that Romilda loves Xerxes. Arsamenes expresses his jealousy.
Scene 2: A bridge joining two shores
Pointing to the bridge he has had constructed to join Asia and Europe, Xerxes sends Ariodates to conquer Europe. Xerxes makes peace with Arsamenes, offering to marry him to Atalanta while he marries Romilda, but the reconciliation breaks down when Xerxes learns that his brother still loves Romilda. Xerxes advises Atalanta to forget Arsamenes, who does not love her, but she is unable to forget him. Xerxes, also, is unable to forget the object of his love.
Elviro looks in vain for Arsamenes.
Scene 3: A quiet place near the town.
Xerxes and Amastre (still disguised) brood on their unrequited loves. Xerxes accepts Amastre into his service, but when Xerxes resumes his suit of Romilda, Amastre warns her again him. He orders her arrested and leaves as she prepares to defend herself. Romilda orders the soldiers to leave Amastre alone. Amastre warns her that while Xerxes is pressing her to become his queen, he is really in love with another.
Scene 1: A gallery
Romilda and Arsamenes discover that others have deceived them. They renew their vows of love and Atalanta resolves to seek love elsewhere. Arsamenes hides as Xerxes appears and continues to press Romilda, who agrees to marry him if her father consents. Xerxes is delighted, but Romilda and Arsamenes bid one another a sorrowful farewell.
Scene 2: A grove
Xerxes repeats his promise to Ariodates of marrying Romilda to an unspecified prince, and Ariodates believes he means Arsamenes. Romilda confesses to Xerxes that Arsamenes had once kissed her and he swears to have him put to death. Romilda begs Amastre to warn Arsamenes, but he reproaches Romilda for her faithlessness
Scene 3: A large temple
Thinking he is following Xerxes' instructions, Ariodates supervises the marriage of Romilda and Arsamen es. Xerxes is furious. He receives a letter from Amastre accusing him of faithlessness. When Xerxes confronts Romilda and Arsamenes, threatening to kill Romilda, they are interrupted by Amastre, who offers to plunge her sword into the faithless heart, but turns it on Xerxes, then revealing her identity. He agrees to return to her, and Romilda and Arsamenes are free to rejoice in their love, while Atalanta repeats her intention of finding love elsewhere.