The Thracian king Demofonte asks the oracle of Apollo how long the practice of the annual sacrifice of a virgin will continue. The answer is puzzling: "as long as the innocent usurper sits on the throne". The nobleman Matusio tries to protect his daughter Dircea from being sacrificed. He and Demofonte are unaware that Dircea is secretly married to Timante, the son of Demofonte and the heir to the throne. Demofonte wants Timante to marry Creusa, a princess of Phrygia. Timante's younger brother Cherinto is accompanying her to the kingdom of Thrace, however he falls in love with her. Meeting Creusa, Timante admits that he can't marry her, but does not explain why.

Dircea has been caught while trying to flee the country and imprisoned, and Demofonte orders the immediate sacrifice of Dircea. Timante tries to release her but with no success. He is also imprisoned. Creusa asks Demofonte for mercy. The king releases Timante and Dircea, and Timante decides to give up the throne in favour of Cherinto.

Suddenly they find a letter revealing that Dircea is the daughter of Demofonte, which makes Timante and Dircea brother and sister. Timante is in despair, and tries to avoid Dircea. However another letter reveals that Timante is the son of Matusio. Everybody is happy. The marriage of Timante and Dircea becomes legal, and Cherinto is the real crown prince and can marry Creusa. No more virgins are sacrificed, since Timante is no longer the "innocent usurper of the throne".

http://en.wikipedia.org/wiki/Demofoonte