Returning to Rhodes, the war-hero Melantius learns that his friend Amintor is to marry his sister Evadne, and not Aspatia, to whom Amintor was betrothed. On the couple's wedding night, after a celebratory masque, Amintor discovers that Evadne has only married him - at the King's command - to conceal the fact that she is the King's mistress. Amintor agrees to maintain the pretence; but Melantius elicits the truth from him, and then prevails upon Evadne to murder the King. He goads Aspatia's father Calianax into surrendering his charge of the fortress to him.
Calianax seeks to expose Melantius's plots to the King at the banquet, but is disbelieved. Evadne, now filled with remorse, murders the King in thier bed. Meanwhile, the jilted Aspatia (disguised as her own brother) provokes Amintor into a duel, in which she is fatally wounded. Evadne, having murdered the King, begs Amintor to take her to his bed, but Amintor rejects her, realising the wrong he has done to Aspatia, whom he still loves. Evadne kills herself, Aspatia revives enough to reveal her true identity, then dies, doubly bereft, Amintor kills himself. Arriving too late upon the scene, Melatius - now pardoned by the new King Lisippus - offers to kill himself too, but is prevailed upon to live.