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.