Xerxes, King of Persia is in love with a plane-tree.
Neither hurricane nor lighting can crush the proud standing tree, whose leaves give peace and tranquillity to the king's heart. Neither hurricane nor lightning…
Arsamenes, Xerxes' brother also visits this place, which is far from the city. He is also visiting his love. It is not a plant, but Romilda, who is singing about yearning lovers and Xerxes' strange affections. The king emerges from his hiding place having immediately fallen in love with Romilda's voice. He orders Arsamenes to propose to the girl for him. His brother's excuses do not alter his decision, and finally he sets out to propose to the girl himself.
Arsamenes manages to meet Romilda before the king arrives. He warns the girl about the danger, and, although his lover swears eternal love to him again - like she has done a hundred times before - Arsamenes foresees a gloomy future. Romilda's sister, Atalanta, however, seizes the opportunity, and in the hope that the king's wish will be fulfilled, confesses her love to Arsamenes immediately - which the lovers, of course, do not take seriously. They do take Xerxes' arrival seriously though. Elviro, Arsamenes' servant warns them in time to hide away, but his pride makes the prince reveal himself. Recognising his own brother as his rival, Xerxes expels Arsamenes and his servant from the court.
Princess Amastris, Xerxes' fiancée is in love with the king, and, dressed as a soldier, follows him to battles and conquests through thick and thin. Xerxes wins one victory after another, mainly thanks to his commander Ariodates, Romilda's and Atalanta's father. In return he promises him that Romilda can marry one of the most prominent members of the royal dynasty, and then sends his troops towards Athens. The loyal Ariodates is grateful for the king's benevolence, and sets out to the war. Amastris, who has eavesdropped Xerxes' secret desires, realises that the king no longer wants to marry her.
Arsamenes sends a note to Romilda with Elviro, in which he invites her lover to a secret rendezvous. Elviro is unwilling to do so as breaking his exile might result in his death. In the meantime Atalanta is trying to persuade her sister to accept the king's proposal and leave Arsamenes. Romilda sees through this naïve idea, and lectures to her sister to stop her futile intrigues as she would never come between her and Arsamenes. Atalanta does not give up so easily, if she cannot achieve her goal simply, she will find another way - she packs her things, and leaves to find her beloved one. She does not get very far.
Elviro tries to carry out his mission disguised as a flower vendor, but Amastris immediately reveals his identity. Elviro tells her the background of his mission, and now the princess can clearly see Xerxes' plans of marriage. Naturally, she breaks down. Eventually, Elviro hands over the note to Atalanta, who promises to forward it to her sister. She obviously does not have the slightest intention to do so, moreover, she exploits the upcoming situation, and when Xerxes arrives she tells him that Arsamenes wrote the letter to him as they have long been lovers. The king has some doubts about this new version, but finally he feels relieved, thinking that it would perhaps be easier to get Romilda. He promises to Atalanta to marry her to Arsamanes. The girl, just in case, warns him: if Arsamenes should deny it, do not believe her.
Xerxes shows the evidence to Romilda triumphantly, but she would still refuse to reject her lover. The king cannot comprehend and interpret this strong emotional attachment, and attempts to start to hate the object of his desires, but he himself does not know how it would be possible. Romilda turns to the audience, and asks them if love can exist without jealousy. In the meantime, the hopeless Arsamenes wants only to die.
Xerxes' troops are gathering at the legendary pontoon bridge over the Dardanelles to cross the strait and invade Greece. The king dispatches his troops with a brief speech then starts to deal with his urgent love affairs. He offers peace to Arsamenes and lets her marry her lover. The misunderstanding, however, soon becomes apparent: his brother refuses to marry Atalanta, since he only loves Romilda. Atalanta is not shocked by the events: she is determined to find a husband, whatever it costs. If not Arsamenes, then someone else.
Elviro tries to alleviate his sorrow over the loss of his master by drinking heavily. Believing that he could find Arsamenes on the other side, he has no choice but to step onto the pontoon bridge, no matter how it pitches and tosses in the storm. The bridge suddenly collapses - at least, Elviro sees it happen -, and he is overwhelmed by seasickness.
Amastris and Xerxes, independently from each other are complaining about their lovers, who are impossible to reach. Before turning to a more serious topic, Romilda arrives. The girl swears to Arsamenes not to change her mind and remain faithful to him, no matter what will happen. Although their jealous row, which is so characteristic of lovers, does not ease, it emerges that they are victims of Atalanta's intrigue.
Xerxes appears suddenly, and Arsamenes and Elviro hide again. When Romilda receives the king, she pretends to be confused in order to gain time, and tries to parry Xerxes' assault by referring to his father's will, as an honourable girl: without his consent any marriage is out of question. Xerxes rushes away happily as he can be certain in his vassal Ariodates' positive reply. Arsamenes obviously does not notice that the girl is completely desperate, misunderstands Romilda's words, and reproachingly demands an explanation for her infidelity.
The retired general is surprised at the king's unexpected visit. Xerxes tells him categorically that today Romilda will have to marry a member of the royal family who is not lower-ranking than himself, the king. The reason for this sophisticated wording is that he, as a king, could not marry a daughter of one of his subjects. Ariodates, of course, misunderstands everything and suspects that Arsamenes, the king's brother will be his son-in-law. And he is most pleased as he has often been a victim of Xerxes' unbridled tyrannical will.
Xerxes now really feels he is in the saddle. He proposes to the girl again, who reveals her secret as a last attempt: her relationship with Arsamenes was more than smere innocent kisses, and now she is no longer worthy of being the king's wife. The furious Xerxes orders the execution of Arsamenes.
Being ardent lovers, Romilda and Arsamenes could quarrel until the end of their lives about who is more faithful than the other or who left the other one. Arsamenes' last hour is approaching, but the never-ending row does not settle until the girl's father appears with a group of priests to let his daughter marry her fiancé. Nobody protests.
Xerxes, who really appears unexpectedly, is shocked to face the fact that Romilda is already Arsamenes' wife. Then a reproachful letter raises his wrath to the extremes, but when he reads the signature, he calls the furies against himself: the letter was written by his fiancée, Amastris.
Everybody tries to calm the furious Xerxes, demanding Arsamenes to kill Romilda. Amastris, still in disguise, undertakes to execute the punishment. On her question whether the sword must drive through the heart of the one who returns true love with infidelity, the king agrees. Then, revealing her identity, Amastris points the sword at Xerxes' chest. The king, of course, regrets his tyrannical behaviour, pardons everyone and praises true love. If, after all this, anyone knows what that actually is.