I, Claudius Season 1 Episode 13

AD 54: Claudius feels that Rome should return to be a Republic but nonetheless marries his niece Agripinilla and makes her son Nero co-heir with his own, teen-aged son Britannicus. Aware of a prophecy that Nero will indeed become sole emperor Claudius encourages Britannicus to flee abroad and then return to restore the Republic but the boy refuses and he is killed by Nero.