METASTASIO'S Argument, reproduced in the London libretto, presents the main lines and the background of the plot in reverse order. ‘Cosroes the Second, King of Persia, having an excessive Tenderness for his younger Son Medarses, A Youth of courtly and deceiving Manners, had a great Mind to make him the Partner of his Crown, by defrauding, with apparent Injustice, his eldest Son Siroes, a Prince valiant in Nature, but impatient of Wrong; who was revenged for it, by the Turn that the anger’d People and the Troops took in his Favour, who loved him infinitely, and rose unanimously in his Behalf.

‘Cosroes, in labouring with his Arms to extend the Confines of his Persian Dominions, was so far advanced in his Conquests towards the East, that he at once deprived Asbites, the King of Cambaja, both of his Kingdom, and of his Life. Of all the Royal Family none could escape the Conqueror's Fury, excepting the Princess Emira, Daughter of the aforesaid Asbites; who, after having travelled a great Way, and being not less induced by Love which she had long since conceived for Siroes, than by the Desire she had to revenge the Death of her own Father, went into the Court of Cosroes, in Man's Habit, under the Name of Hydaspes: And there always dissembling her Hatred, and unknown to every Body but Siroes, was by him himself introduced; and knew so well how to insinuate herself into the Favour of Cosroes, That she (or he rather under that borrow’d Name) became his most darling and bosom Confident. On these Foundations drawn partly from the Writers of Turkish History, and partly too from Passages probably imagin’d, The Events of this Drama take their proper Turn. The Scene is in the City of Seleucia.’

Act I. A spacious Temple dedicated to the Sun, with an Altar adorned with the Image of that Deity. Cosroe, about to settle the succession, orders both his sons to swear to abide by his decision.