Place: Abydos, Persia
Time: about 470 BC
Act 1
A garden with a large plane tree and a summerhouse on the side
The King of Persia, Serse, gives effusive, loving thanks to the plane tree for furnishing him with shade.(Arioso:Ombra mai fu). His brother Arsamene, with his buffoonish servant Elviro, enters, looking for Arsamene's sweetheart Romilda. They stop as they hear her singing from the summerhouse. Romilda is making gentle fun of Serse with her song. He is in love with a tree, but the tree does not return his affection. Serse does not know that his brother is in love with the singer, and entranced by her music, Serse announces that he wants her to be his. Arsamene is horrified when Serse orders him to tell Romilda of his love. Arsamene warns Romilda of what Serse wants - this encourages Atalanta, Romilda's sister, who is secretly in love with Arsamene also and hopes that Romilda will be Serse's and then she can have Arsamene.
Serse tells Romilda that he wants her for his queen and when Arsamene remonstrates Serse banishes him. Romilda is determined to remain faithful to the man she loves, Arsamene.
Outside the palace
Princess Amastre now arrives, disguised as a man. She was engaged to Serse but he jilted her and she is furiously determined to be revenged.
Ariodate, general to Serse and father of Romilda and Atalanta, enters with news of a great military victory he has won. Serse is grateful to him and promises that as a reward his daughter Romilda will marry a man equal in rank to the King himself.
Arsamene gives Elviro a letter for Romilda, telling her how distressed he is at their forced separation and pledging to try to visit her in secret. Romilda's sister Atalanta, hoping to secure Arsamene for herself, tells Romilda that Arsamene is in love with another girl, but Romilda does not believe it.
Act 2
A square in the city
Elviro has disguised himself as a flower-seller in order to deliver his master Arsamene's letter to Romilda, and is also putting on a rural accent. He does not approve of the King's desire to marry a mere subject such as Romilda and makes this clear. Princess Amastre, in her disguise as a man, hears Elviro expressing this and she is aghast at the King's plan to marry another when he promised to be hers (Aria:Or che siete speranze tradite).
Amastre leaves in despair and rage and Atalanta enters. Elviro tells her he has a letter for her sister and Atalanta takes it, promising to give it to Romilda. Instead she mischievously shows the letter to the King, telling him that Arsamene sent it to her and no longer loves Romilda. Serse takes the letter and shows it to Romilda, telling her Arsamene is now in love with Atalanta, not her. Romilda is shaken (Aria:E’ Gelosia).
Princess Amastre has decided to kill herself but Elviro arrives in time to stop her. She resolves to confront the King with his ill-treatment of her. Elviro tells Amastre that Romilda now loves Serse: Amastre is devastated (Aria:Anima infida).
By the newly-constructed bridge spanning the Hellespont and thus uniting Asia and Europe
Sailors hail the completion of the bridge, constructed under Serse's orders, and Serse orders his general Ariodate to cross the bridge with his army and invade Europe.
Serse encounters his heart-broken brother Arsamene and tells him to cheer up, he can marry the woman he now loves, Atalanta, no problem. Arsamene is confused and insists he loves Romilda, not Atalanta. Hearing this, the King advises Atalanta to forget about Arsamene, but she says that is impossible.
Elviro watches as a violent storm threatens to destroy the new bridge. He calms his nerves with drink.
Outside the city in a garden
Serse and Arsamene are both suffering from jealousy and the tribulations of the love lorn. Serse again implores Romilda to marry him but she remains firm in her refusal. The violently furious Amastre appears and draws a sword on the King but Romilda intervenes. Amastre says Romilda should not be forced to marry a man she does not love, and Romilda praises those who are true to their hearts (Aria:Chi cede al furore).
Act 3
A gallery
Romilda and Arsamene are having a lovers' spat about that letter, but calm down when Atalanta appears and admits her deception. She has decided she will have to find another boyfriend somewhere else.
Serse again implores Romilda to marry him and she tells him to seek her father's permission, if he consents, she will. Arsamenes bitterly reproaches her for this(Aria:Amor, tiranno Amor).
Serse once more asks Ariodate if he is happy for his daughter Romilda to marry someone equal in rank to the King. Ariodate thinks Serse means Arsamene and happily gives his consent. Serse tells Romilda that her father has agreed to their marriage but Romilda, trying to put him off, tells him that Arsamene loves her and in fact he has kissed her. Serse, furious, orders his brother to be put to death.
Amastre asks Romilda to take a letter to the King, telling her that this will help her. Amastre bewails her plight, having been abandoned by Serse, who promised to be hers (Aria:Cagion son io).
Arsamene blames Romilda for the fact that he has been sentenced to death, and the lovers again quarrel (Duet:Troppo oltraggi la mia fede).
The temple of the sun
Arsamene and Romilda have been summoned to the temple and they come in, still quarreling, but they are amazed and overjoyed when Ariodate tells them that Serse has agreed to their wedding and he marries them then and there.
Serse enters, ready to marry Romilda, and is enraged when he discovers that it is too late, Ariodate has married his daughter to Arsamene. Serse bitterly denounces Ariodate for that and is even more enraged when a letter arrives, apparently from Romilda, accusing him of faithlessness. When he discovers that the letter is actually from his previous fiance Amastre, whom he jilted, his fury only increases (Aria:Crude Furie degl' orridi abissi).
Serse takes his sword and orders Arsamene to kill Romilda with it; but Amastre interrupts this and asks Serse if he truly wants treachery and infidelity to be punished. Serse says he does whereupon Amastre reveals her true identity as Serse's betrothed. Serse, abashed, admits his fault - he will marry Amastre as he promised, he wishes his brother Arsamene and Romilda happiness in their marriage, and all celebrate the fortunate outcome of events (Chorus:Ritorna a noi la calma).