[1] In the epistle dedicatory to the original libretto, Busenello explains the plot as follows: The king of Armenia, in concert with other Asian princes, engages in bloody battle with Darius III of Persia, and kidnaps his wife Parisatidis and daughter Stateira.
An occasion such as this could not procure any other effect than that the two fall in love; this the princess does, giving him signs of her ardent passion.
Wherein: Stateira, being a young woman and therefore incapable of keeping anything secret, speaks of her love to a servant girl, known as Ermosilla, but who is actually Usimano, an Egyptian prince.
No sooner has Usimano learnt of Stateira's love for the Arab than he is seized with jealousy against him; this anger is the basis for all the drama, which is only resolved by a series of events as you shall see.
Floralba, Stateira's other servant girl, is in love with the Arab king and only later on finds out that she is his sister.