The minstrel Carlo Broschi has hidden his sister Casilda in a convent to protect her from the machinations of the clergy who wish to make a present of her for King Ferdinand VI. In Carlo's opinion, she is in love with an unknown cavalier- likewise too highborn to have any lawful intentions toward her.
Carlo happens upon the King, who is possessed by melancholy and succeeds in cheering him with a song (It was, in fact, Ferdinand's predecessor Philip V, for whom Farinelli was engaged as a music therapist). As a reward, he is invited to the court, where he encounters his sister's lover, Raphael d'Estuniga. Raphael is so despondent over his thwarted passion that he is ready to sell his soul, so Carlo introduces himself as Satan, ready to lend aid for half of his takings.
Casilda appeals to Carlo for protection; she has been kidnapped by the priests and brought to the king, who, only recently having recovered his sanity, takes her for a ghost. Carlo leaves to speak with the queen (Louise-Rosalie Lefebvre created this roleLouise-Rosalie Lefebvre) and leaves the lovers alone. Raphael, who has obtained an office due to Carlo's influence and has had uncommon luck at gambling, is so confident of supernatural aid that he is nonplussed at the king's entrance, even when the latter orders his death. Carlo attempts to smooth things over by telling the king Raphael is her husband, but the Grand Inquisitor exposes the fabrication, enraging the king against Carlo as well. Things can only be put right by Carlo's revealing all and reminding the king that the queen still suspects nothing. Carlo, who has never hesitated to claim his 50%, tells his future brother-in-law that his share will be Casilda's happiness this time.
