Mitridate is the King of Pontus, an Hellenic kingdom on the Black Sea. He and his army have battled the invading Romans, but have been defeated and Mitridate is presumed dead. He had left his betrothed, Aspasia, in the care of his two sons, Sifare and Farnace, whose political loyalties are divided; Sifare fought alongside his father, but Farnace supported the Romans. Both sons have fallen in love with Aspasia. She seeks Sifare’s help in avoiding unwelcome advances from Farnace, suggesting that she might indeed be attracted to Sifare. Mitridate unexpectedly returns, bringing Farnace’s fiancée, lsmene, an East Indian princess. Mitridate insists on an immediate marriage between Farnace and Ismene until he learns of Farnace’s dual treachery – coveting his own fiancée, Aspasia, and conspiring with the enemy. He vows bloody vengeance.

Ismene tries to convince Farnace not to leave her, then appeals to Mitridate, who suggests that Sifare would be a better husband. Mitridate orders his own immediate wedding to Aspasia, but when she hesitates, he suspects she favors Farnace. Aspasia and Sifare realize they must suppress their feelings for one another. Mitridate tricks Farnace into revealing his relationship with the enemy. Farnace takes revenge by denouncing Sifare’s love for Aspasia, whereupon Mitridate dupes Aspasia into admitting her love for Sifare. Enraged, Mitridate imprisons all three and condemns them to death.

Mitridate, spurned by Aspasia, prepares for battle. A soldier brings Aspasia poison, a gift from Mitridate. Sifare, freed from prison by lsmene, arrives in time to save her before going to fight at his father’s side. Farnace too is released from prison, but by Roman soldiers who want him to take the throne. He decides instead to earn his father’s forgiveness by rejecting Aspasia and joining the fight against the Romans. Mitridate, having thrown himself on his sword rather than be captured, begs Aspasia’s forgiveness and blesses her union with Sifare. Learning that Farnace has been responsible for burning the enemy fleet, Mitridate forgives him too. The brothers join lsmene and Aspasia in pledging resistance to Roman tyranny.