10 Biggest Doctor Who Conspiracy Theories
9. The Doctor And The Master Are Siblings
The relationship between the Doctor and the Master has always been a complex one. They disagree on many things, but nevertheless respect one another. They know each other better than anyone else in the universe. They’ve had beef since childhood, and always will do.
If you didn’t know better, you might say they were siblings.
The idea that the Master is related to the Doctor has been alluded to, and almost confirmed, on several occasions. It was one of the plot twists considered for Jon Pertwee’s final story – which, had it not been for Roger Delgado’s untimely death, would've constituted one last showdown between the Third Doctor and the Master.
Elsewhere, though the Master's final words are cut off in Planet of Fire, producer John Nathan-Turner has suggested that he was going to call himself the Doctor's brother.
The revived series has entertained the idea, but taken it less seriously. In The Sound of Drums, when Martha asks the Doctor if the Master is his “secret brother”, he tells her she’s been watching too much TV. RTD's not-so-subtle thoughts on this particular theory.