Like the Doctor, the Master is also a Time Lord from the Planet Gallifrey. Unlike the Doctor, the Master went quite mad when as a child he was exposed to the Untempered Schism. This madness has given rise to many attempts at ruling the universe. Fortunately the Doctor has been around to put a top to the evil Master's schemes.

Without a doubt, the Master is the Doctor's greatest nemesis, a match for the Doctor in intelligence and resourcefulness. The Master has come close to destroying the Doctor many times, but the Doctor usually manages to prevail.