Act I

Mitridate, King of Pontus, has reportedly been killed in his battles with the Romans. During his absence, his sons—Sifare, a loyal supporter of his father’s cause, and Farnace, who has aligned himself with Rome—have remained behind. Both have, however, fallen in love with their father’s fiancée, Aspasia, who has been entrusted to their care. Aspasia turns to Sifare for help resisting Farnace’s persistent advances, revealing her inclination toward Sifare instead. Sifare, noble and conflicted, is burdened by loyalty to his father.

The unexpected return of Mitridate, who isn’t dead after all, throws the household into disarray. Not only is he alive, but he brings with him Princess Ismene, daughter of his ally, the King of Parthia, who has been promised to Farnace. Mitridate hopes to cement political alliances by having them marry without delay. Farnace demurs, and Mitridate soon uncovers evidence of Farnace’s conspiring with Rome and his coveting of Aspasia. Outraged, Mitridate declares vengeance.

Act II

Ismene confronts Farnace and urges him to honour his betrothal, but he coldly rejects her. Devastated, she turns to Mitridate, who suggests Sifare as a more suitable husband for her. Mitridate also proposes his immediate wedding to Aspasia, hoping to restore personal and political order. Aspasia hesitates, and her reluctance deepens Mitridate’s suspicions, though he remains unaware of her secret affection for Sifare.

Aspasia and Sifare realise they must hide their feelings for each other. Mitridate, still wary, schemes to expose further treachery. He tricks Farnace into revealing his correspondence with the enemy. Cornered, Farnace responds by accusing Sifare of loving Aspasia. Mitridate turns his suspicion on Aspasia and manipulates her into confessing her love for Sifare. Enraged and feeling betrayed by his family, Mitridate imprisons Sifare, Farnace and Aspasia, condemning them to death.

Act III

Mitridate prepares for battle. Aspasia prepares to take poison, but Sifare, who has been freed by Ismene, arrives in time to stop her. He chooses to join his father in battle, proving his loyalty despite everything.

Farnace, released by Roman agents hoping to place him on the throne, refuses their offer and instead destroys the Roman fleet, seeking redemption. Wounded and unwilling to be captured, Mitridate takes his own life. In his final breath, he forgives Aspasia, blesses her union with Sifare, and reconciles with Farnace, who now agrees to marry lsmene. They together agree to stand against Roman tyranny.