ABRIEF NOTE To the Reader takes the place of the usual Argument. ‘The contexture of this Drama is so very easy, that it wou’d be troubling the reader to give him a long argument to explain it. Some imbicillities, and the temerity of Xerxes (such as his being deeply enamour’d with a plane tree, and the building a bridge over the Hellespont to unite Asia to Europe) are the basis of the story; the rest is fiction.’ This is a slight understatement. Herodotus deals with the plane tree incident and the Hellespont bridge in Book VII of his History (chapters 31, 33 and 34). In Book IX (chapter 108 et sequ.) Xerxes falls in love with his brother's wife. The intrigue takes a more savage course than in the opera, and Amestris (Amastre) plays a far from edifying part. The printed libretto and Handel's autograph are both rich in stage directions. They mostly complement each other and are amalgamated in the following plot summary (though the scene numbering differs), except as noted.

The curtain rises on A summer house on one side of a garden, in the middle of which is a plane tree, and Xerxes under it. The King of Persia craves a blessing on the tree, which he stands and admires with rapture. His brother Arsamene enters in search of his beloved Romilda, accompanied by his servant Elviro, who looks drowzy. A short symphony is heard, and presently Romilda's voice in the summer house; she mocks the King's love for a tree, which can give him no more tangible return than the murmur of its leaves. Serse, captivated by the voice and hearing his name mentioned, at once has designs on the singer and cross-questions Arsamene about her identity. Arsamene prevaricates, pointing out that it is unlawful for a king to marry anyone but a queen. Serse, after vainly ordering his brother to plead his cause, goes off to do it himself. Arsamene, confident of Romilda's love, warns her of Serse's intention, thereby giving secret hope to her sister Atalanta, who also loves Arsamene and looks at him tenderly making big eyes [molti vezzi].