The Master Brain is the main antagonist of the Doctor Who story "The Mind Robber", a computer with ambitions to rule Earth.
The Master Brain was a super-computer which controlled the Land of Fiction, a realm where fictional characters were real. At some point, it abducted a writer from 1920s England and linked him to it, making him the Master of the Land of Fiction. With the Master's help, the Master Brain was able to direct the actions of any of the characters that populated the Land, which it policed with its White Robots and Clockwork Soldiers.
When the TARDIS dwelled close to its domain, the Master Brain managed to draw the Doctor, Jamie and Zoe into its Land, hoping to turn them into fiction by sending a variety of threats against them. The Doctor saw through the traps, refusing to respond to the fictional threats as if they were real. Finally, he was brought before the Master Brain and offered a chance to replace the Master but refused.
The White Robots successfully turned Jamie and Zoe into fiction, after which the Master had them lure the Doctor into a trap, connecting him up to the Master Brain. The Master explained the Master Brain's plan to gain control of Earth and transport its population to the Land, making them fiction. However, being connected in this way gave the Doctor access to the Master Brain's power, allowing him to free his friends and control the fictional characters to protect them. Enraged, the Master Brain ordered the White Robots to kill the Doctor. However, Jamie and Zoe managed to sabotage the Master Brain's settings. The damage was compounded when the White Robots fired wildly at the computer and the Master Brain was destroyed.