Xerxes calms his soul by singing of his love for a plane tree. Elviro and Arsamene, Xerxes' brother, go to see his beloved Romilda. When Xerxes sees Romilda he falls in love with her immediately. He finds out from Arsamene who she is, and orders him to arrange their marriage. Arsamene hesitates, so Xerxes decides to proclaim his love for himself. Arsamene warns Romilda about Xerxes. Romilda's sister Atalanta declares her love for Arsamene. He ignores her. Xerxes asks Romilda to share the throne with him. When Arsamene tries to intervene he's banished by his brother. To the frustration of the King, Romilda swears to remain faithful to her beloved. Amastre, daughter of the King of Tagor, disguised as a soldier, observes her fiancé Xerxes trying to woo Romilda. Amastre loves him and wants to win him back. General Ariodate, Romilda and Atalanta's father, returns from victory at war. To reward him Xerxes promises that Romilda will marry a member of the royal family. Xerxes goes into raptures about his new love, without naming Romilda. Arsamene sends Elviro from exile with a letter for Romilda, asking for a secret meeting. Amastre, desperate, wanders around the royal court, swearing to take revenge on Xerxes' infidelity. Atalanta tries everything to win the affections of Arsamene, her sister's intended. She tells Romilda she should marry Xerxes. But Romilda sees through her plot. Elviro disguises himself as a flower seller to smuggle Arsamene's letter to Romilda. He meets Amastre (in soldier's clothes) and tells her about the Xerxes, Romilda, Arsamene love triangle. The abandoned woman gives up all hope. Atalanta manages to persuade Elviro to hand over Arsamene's letter, telling him that Romilda has accepted the King's proposal of marriage. While Atalanta's reading the letter she's interrupted by Xerxes. Atalanta maintains that the letter was addressed to her and that Arsamene's only pretending to love Romilda. Xerxes is overjoyed and gives his blessing to Atalanta's marriage to Arsamene. Xerxes shows Romilda the letter. Although shaken, she swears to remain true to Arsamene. Xerxes storms off in a rage. The bridge that Xerxes built across the sea is officially opened. Arsamene, in the meantime, has returned from exile, plagued by thoughts of suicide. Xerxes promises him his beloved (he means Atalanta) and suggests a double wedding, but Arsamene only wants Romilda. The king tells Atalanta that Arsamene doesn't love her. INTERVAL Elviro gets drunk while looking for Arsamene and sees how the new bridge breaks up in a storm. Xerxes and Amastre bemoan their fates. Amastre almost gives herself away but manages to keep up her disguise and hides. Xerxes tries to win Romilda again, but she's not to be persuaded. Amastre intervenes, wanting to save Romilda. The king has »him« arrested, but Romilda makes sure that »he« is set free. Arsamene and Romilda ask Atalanta to explain her intrigues. The lovers are reconciled. Atalanta has no choice but to find a new man. Xerxes tries to talk Romilda round a third time. To play for time she appears to give in, on condition that her father Ariodate gives his blessing. Arsamene's heard everything and takes her to task. Romilda insists that she's innocent. Xerxes repeats his promise to Ariodate that Romilda will have a royal bridegroom. The delighted father believes that he means Arsamene. Romilda's resolute and rejects Xerxes again. He orders that Arsamene be killed. Romilda begs Amastre to warn Arsamene. In return Romilda's to give Amastre's declaration of love to Xerxes. Romilda warns her beloved that he's in danger. Arsamene doesn't trust her. They argue. In the midst of the wedding preparations Ariodate tells them of Xerxes' promise, according to which they are to marry. When Xerxes reveals the true identity of the „royal bridegroom“ (he means himself) Ariodate explains that Romilda's just married Arsamene. After Xerxes receives Amastre's love letter he calls in desperation for furies to help and orders Arsamene to kill Romilda. Amastre intervenes, reveals her true identity and threatens to kill Xerxes and then herself. Driven into a corner, Xerxes is remorseful and asks everyone to forgive him. Two couples are reunited: Romilda – Arsamene and Amastre – Xerxes. Atalanta's still searching. Peace is restored at the royal court.