Fan CastingThe Master

Like the Doctor, the Master fled from Gallifrey in a TARDIS of his own, and, having fully embraced his darker nature, would go on to pit himself against the Third Doctor and UNIT during the Doctor's exile on Earth. Later, having expended his original regeneration cycle, the Master survived in the ghastly form of a living cadaver, in which form he fought the Fourth Doctor, before exploiting the powers of the Source on Traken to steal the body of Tremas. However, Tremas' body began to age and became less usable under the effects of the Cheetah virus. Eventually, the Master was forced to dispose of the body after Tremas' mind battled against his under the effects of the Mind Wrest.