THE background of the opera is the (mythical) foundation of the city of Naples. According to the Argument in the printed libretto ‘Parthenope, the Daughter of Eumelius, King of Phera in Thessaly, departed from Calcis [sic] in the Isle of Euboea, now call’d Negropont, to follow the Augury of a Dove, and upon the shore of the Tyrrhene Sea, founded the City Parthenope, now call’d Naples. This is mention’d in the Eleventh Chapter of the first Book of the History of the City and Kingdom of Naples, by Gio. Antonio Sumonte. The rest of the Drama is fictitious.’

Act I. Sunrise. Part of a City near the Sea, adorned with great Solemnity; in the Middle an Altar, with the Statue of Apollo. Priests and Nymphs with Basons in their Hands full of Laurel Leaves. Parthenope on a Throne, Arsace and Armindo. Partenope invokes Apollo to favour her new city; the chorus call down a blessing on her. The Fire kindles suddenly on the Altar, and on Partenope's command they burn the Laurels. To them Rosmira in the Habit of an Armenian (she is described in the cast list as ‘Princess of Cyprus, in love with Arsace, and promis’d to him, but afterwards forsaken by him’). She kneels and tells Partenope that she is Eurimenes King of Armenia, shipwrecked off the coast. Partenope promises hospitality. Ormonte, Captain of Partenope's Guards, introduces a Messenger and reports that the hostile people of Cumae have captured ‘the neighbouring Mountain and the Plain’ and their prince Emilio asks for conference. Partenope seems pensive, though Arsace, ‘Eurimene’ and Armindo, Prince of Rhodes, try to encourage her. She sends the messenger to summon Emilio and goes out with Ormonte, followed by Arsace, who as he retires, looks at Rosmira, puzzled and disturbed by her appearance as Eurimenes. Armindo tells Rosmira that he loves Partenope, but has not told her since she and Arsace have sworn eternal fidelity. Rosmira, concealing her indignation at Arsace's treachery, urges Armindo to speak out in his own interest. He resolves to do so. In a Royal Hall Rosmira confronts Arsace: she abandoned all to follow him.