Scene: Naples, in antiquity

Act 1
In the first scene we see Queen Partenope, founder of Naples, in her throne room adorned with a statue of Apollo, entertaining her guests, among them handsome and dashing Prince Arsace of Corinth, suitor for her hand, and the rather shy and lacking in confidence Prince Armindo of Rhodes, who is also in love with Partenope but cannot bring himself to tell her so. A new guest arrives and introduces "himself" as Prince Eurimene, but "he" is really the princess Rosmira in disguise, Arsace's previous fiancé whom he jilted when he decided to try to win the hand of Queen Partenope and who has tracked him down to Naples. Arsace is astounded by the resemblance of the newcomer "Eurimene" to his ex-girlfriend Rosmira and confronts her when they are alone. Rosmira admits it is she and berates him for his inconstancy. Arsace claims he still loves her, to which Rosmira replies that if so he can prove it by promising not to reveal her name or that she is a woman. Arsace solemnly swears to do so.

"Eurimene" /Rosmira also has a conversation with the sad Armindo whom she discovers really loves Partenope for herself, rather than just being after her position and money. Armindo can't bring himself to tell Partenope that he loves her because he thinks she prefers Arsace. Partenope herself has also noticed Armindo moping about and wants to know what the matter with him is. He does confess his love to her, to which she replies that she is committed to Arsace. Hearing this, Rosmira steps forward in her disguise as "Eurimene" and says "he" loves her too, hoping to keep Partenope from Arsace, but upsetting Armindo in the process.

Yet another suitor for Partenope's hand arrives, Prince Emilio of the neighbouring kingdom of Cumae. He has brought his army with him, and demands she agree to marry him. She refuses and he threatens her with war, to which she replies that she will not be intimidated. She says she will go into battle and fight herself and asks Arsace to lead her troops, which makes the other suitors jealous.

Alone with "Eurimene", Armindo is feeling very down about the fact that "Eurimene" is his rival, but "Eurimene" assures him that is not really the case.

Act 2
The second act begins with a depiction onstage of the battle between Partenope's and Emilio's forces. Partenope is ultimately victorious,and Emilio is imprisoned, but not before Armindo has rescued her from a dangerous situation and thereby saved her life. Rosmira, in her identity as "Eurimene", also fought in the battle and proudly asserts that "he" captured Emilio. Arsace denies this, claiming credit for that himself. At this "Eurimene" challenges Arsace to a duel.

Armindo once again tells Partenope how much he loves her. She is more encouraging to him this time but does not commit herself.

Arsace tries to make things up with Rosmira but she is very disdainful towards him. Arsace is torn between his desire for Queen Partenope and his old love Rosmira.

Act 3
Rosmira, still in disguise as "Eurimene",tells Queen Partenope that "he" challenged Arsace to a duel because "he" was sent by Princess Rosmira to avenge her honour on the man who promised to marry her but jilted her at the altar. The horrified Partenope asks Arsace if this is true and he has to admit it is. Partenope declares she will have nothing more to do with him; Armindo is looking better and better to her as prospective husband.

Arsace is having a sleepless night, knowing that he will be expected to fight "Eurimene" in a duel the next day. But how can he fight a duel with the girl he loves? Rosmira comes to him in his rooms, watched by Partenope. When Arsace calls Rosmira's name, Partenope steps forward accusing him of faithlessness, and Rosmira /"Eurimene" does the same.

The next morning, all assemble for the duel between Arsace and "Eurimene". Arsace is in a dilemma as he has solemnly sworn not to reveal Rosmira's identity but he cannot bring himself to fight a duel with a girl. Suddenly seized by an inspiration, he says that as the person who was challenged, he is allowed to decide how the duel will be fought - and demands they fight without their shirts on. "Eurimene" hesitates, but all the others tell "him", yes, that is correct, Arsace can say how the duel will be fought, so go on, take your shirt off. "Eurimene" has no choice but to admit "he" is really Princess Rosmira and accepts the contrite Arsace as husband. Partenope will marry the overjoyed Armindo and Emilio will take his troops and go back home to Cumae.