Arbate, the governor of Nymphæum, welcomes Sifare. We learn that Sifare resents his brother, Farnace, because of his brother’s strong ties with their enemies, the Romans. Arbate pledges his loyalty to Sifare. Aspasia pleads for Sifare to help her against advances by Farnace. He accepts her plea and reveals his love for her. Farnace makes his advances on Aspasia. Aspasia refuses with support from Sifare who protects her from his forceful brother. News arrives that Mitridate is alive and is approaching the city. Arbate urges brothers to conceal their differences and greet their father. Brothers agree to hide their feelings for Aspasia. Farnace conspires with Marzio, Roman legionary officer, against Mitridate. Mitridate arrives on the shores of Nymphæaum with princess Ismene, daughter of his ally the King of Parthia. Mitridate wants Farnace to marry Ismene, his promised bride. Ismene is in love with Farnace but senses problems and is worried about her future. Arbate tells Mitridate that Farnace is pursuing Aspasia not mentioning Sifare. Jealous Mitridate swears revenge on Farnace. Farnace scorns and threatens Ismene. She tells Mitridate who suggests that she should marry Sifare. Mitridate asks Aspasia for immediate marriage but she hesitates proving to him she is unfaithful. Aspasia confesses love to Sifare but they both agree to part to save their honour. Sifare plans to leave and Aspasia is troubled with the conflict between love and duty.
Mitridate is aware of Farnace's plot against him with the Romans and plans his revenge despite Marzio’s offer of peace. He arrests Farnace to execute him. Ismene rescues the prince who admits treachery but implicates Sifare. Mitridate tricks Aspasia into admitting her love for Sifare and swears revenge. Aspasia and Sifare wish to die together in fear of Mitridate’s threats. Ismene, still in love with Farnace, tries to convince Mitridate to forgive Aspasia. Romans attack and Mitridate leaves for battle. Aspasia contemplates suicide by poison. Sifare also wants to die and joins his father in the battle. Marzio liberates Farnace and promises the rule of Nymphæum to him. Farnace changes his mind deciding to side with Mitridate. Mitridate commits suicide avoiding defeat. Before he dies he gives his blessing to Sifare and Aspasia and forgives Farnace who now agrees to marry Ismene. All four pledge to free the world from Rome.