Partenope asks for Apollo’s blessing. Rosmira enters, disguised as a man, and introduces herself as Eurimene. Despite the disguise, Arsace recognizes her as his former betrothed; before they can talk, Ormonte announces that Emilio wants to meet Partenope. Armindo confides in Eurimene that he loves Partenope; she, however, is in love with Arsace. Rosmira scolds Arsace for abandoning her; he begs her forgiveness and she insists that he must not reveal her true identity. Armindo admits to Partenope that he loves her. When Eurimene interrupts Partenope and Arsace, ‘he’ declares himself to be yet another of Partenope’s suitors. Partenope continues to assert her devotion to Arsace. Emilio offers Partenope marriage; when she refuses, he threatens her. She asks Arsace to lead her forces against Emilio, and this provokes jealousy among the other suitors. Armindo is perturbed by Eurimene’s feelings for Partenope, but Eurimene reassures him that his affections lie elsewhere.
Partenope’s and Emilio’s forces engage, and Emilio is captured by Arsace.
Eurimene claims all credit for Emilio’s capture and Arsace says nothing to the contrary. When Emilio contradicts Eurimene’s assertion, Eurimene challenges Arsace to a duel to prove his honour. Arsace attempts to pacify Eurimene whose feelings are torn between love and rage. Armindo declares his love for Partenope; she, however, continues to want only Arsace. Alone together, Rosmira and Arsace struggle with conflicting emotions.
Eurimene tells Partenope that he challenged Arsace not for himself but on behalf of a woman – Rosmira – whom Arsace had promised to marry and then abandoned. When Arsace admits the allegation is true, Partenope rejects him and gives hope to Armindo. Emilio offers Arsace his support in the forthcoming duel. Arsace asks for Rosmira’s forgiveness. When Partenope discovers them together, Rosmira manages to conceal her true identity. Both women scorn Arsace, who rails at the tyranny of love. The contestants are given their weapons for the duel. When Arsace suddenly demands that he and Eurimene must fight bare-chested, Eurimene is placed in a dilemma and chooses to reveal ‘his’ true identity. The lovers change partners. Partenope takes Armindo and Rosmira and Arsace are reunited.