Serse, headstrong and rich businessman, finds peace of mind at the sight of a tree. He is engaged to Amastre. Tired of her, he soon chooses Romilda, whose voice has enchanted him, as the new object of his desire. He wants to possess her at all costs and orders Arsamene to arrange the union.

Arsamene, Serse's brother, has recently started dating Romilda and initially kept his love a secret. Since he does not carry out Serse's order because, as Serse's rival, he does not want to let go of Romilda, Serse withdraws his right of residence.

Romilda remains faithful to Arsamene. The aberrations and confusions of love do not lead her astray.

Atalanta, Romilda's sister, loves Arsamene and wants to steal him away from her sister and win him over by any means necessary. If necessary, with lies and intercepted letters. She tries to convince Romilda of Serse's merits. 

Amastre, Serse's repudiated fiancée, continues to seek Serse's proximity under a different identity. She hopes that he will soon return to her in repentance.

Ariodate, father of the two sisters, is only too willing to marry Romilda to someone from Serse's family. He thinks Serse wants Romilda to marry Arsamene and marries the two off - to Serse's great dismay.

Elviro, Arsamene's best friend, tells Amastre the whole story of the Serse-Romilda-Arsamene love triangle. He fatally hands over a love letter from Arsamene to Romilda to Atalanta.