An immensely powerful super-computer whose origins are obscure. It controlled a domain outside normal space and time where fictional characters could exist. This was generated through the harnessed imagination of a captive human - a writer of boys' adventure stories, abducted from early 20th Century Earth. As a mortal human being, he could not maintain the Land of Fiction forever, and so the Master Brain sought out the Doctor to take his place. Its ultimate goal was to transfer the entire population of Earth into this domain, leaving the planet empty and open to exploitation for itself or its unknown creators. When the Doctor prevented this from happening - using fictional characters as weapons to defend himself and his companions, Jamie and Zoe - the Master Brain ordered its White Robot servants to destroy them. When Zoe began overloading the computer by randomly pressing every control, the Robots tried to stop her. They fired upon the Master Brain itself - destroying it in the crossfire.
Appearances: The Mind Robber (1968).