"The Master" - Also known known by various other aliases — was a renegade Time Lord, serving as both a friend and an opponent of the Doctor.
Though they had been friends from childhood and schoolmates at Prydon Academy, the Master's lust for power would eventually pull the two apart. While the Doctor and the Master would remain close friends despite their rivalry, the Master developed an intense yhatred for and often sought to kill the Doctor. Despite this enmity, however, the two would on occasion act as allies.
Like the Doctor, the Master would also flee from Gallifrey in a TARDIS of his own, and fully embrace his darker nature. The Master then went on to fight various battles against the Third Doctor and UNIT during the Doctor's exile on Earth, even spending a short period also trapped on the planet after his dematerialisation circuit was briefly stolen by the Doctor and when he was imprisoned on Fortress Islandafter being arrested by UNIT, until he was able to orchestrate his escape with the aid of the Sea Devils.
