The setting is the city of Nymphaeum around 63 BC. King Mitridate of Pontus is believed to have been killed in battle against the invading Romans. In his absence, his two sons, Farnace and Sifare, both vie for the love of his fiancée, the Greek princess Aspasia. Sifare remains loyal to his father and resists the Romans, while the treacherous Farnace openly sides with them.

Mitridate unexpectedly returns, having faked his death to test his sons’ fidelity. He quickly learns of Farnace’s betrayal and his sons’ rivalry over Aspasia. A web of lies, jealousy, and political conspiracy unfolds. Mitridate, determined to punish the traitors and secure his kingdom, faces both external threats from the Romans and internal conflict within his own family. Ultimately, Farnace repents, and in the final, climactic act, Mitridate, realizing his cause is lost and preferring death over capture, kills himself. He urges Sifare and Aspasia to unite, leaving Sifare to carry on the fight against Rome, thereby elevating loyalty and true love above political ambition.