Pharnaces and Xiphares are both in love with their father’s betrothed, Aspasia. Aspasia, meanwhile, is in love with Xiphares. Pharnaces conspires with the Roman Marcius against his father, Mithridates. Meanwhile, Mithridates, rumoured dead in his struggle against the Romans, returns, fearing the disloyalty of both of his sons. Arbates reassures him of the loyalty of Xiphares. Mithridates, now about to renew his warfare against Pompey, distrusts Aspasia and imprisons both his sons when Pharnaces reveals the love of Xiphares for Aspasia, although Xiphares had honourably decided to leave Aspasia and Pontus. Mithridates then sends Aspasia \ poison, which Xiphares stops her drinking. In battle the King is victorious but wounds himself mortally, returning to unite Aspasia and Xiphares and to forgive Pharnaces, who has acted against the Romans.