Act 1

Pontus, on the Black Sea, 63BC. King Mitridate has gone away to war, leaving his sons Sifare and Farnace as his regents while he is abroad. Farnace hears a false rumour that his father has died, and declares his love to the king's new betrothed, Aspasia. Aspasia asks Sifare to protect her. Mitridate returns from war with the princess Ismene as Farnace's bride. He learns of Farnace's plan marry Aspasia, and determines that he must die.

Act 2

Aspasia and Sifare declare their love for each other. Aspasia is in despair because she is engaged to be married to Sifare's father the king.

Act 3

Farnace has plotted with his father's enemies, the Romans, and his plot is uncovered. Farnace reveals that Sifare is his father's real rival. Mitridate deceives Aspasia into declaring her love for Sifare, and condemns them both to death. Ismene placates the king for long enough for Sifare and Farnace to defeat an attack by the Romans. Mitridate is mortally wounded in the battle, but in the moments before his death he rejoices at his sons' loyalty and their future marriages.