The Master Computer was a vast and ancient computer at the center of a Robot civilization discovered by the crew of the USS Enterprise in 2266. The Master Computer was housed in an expansive chamber in the Robots' city, beneath a gigantic statue of one of the Robots' humanoid creators. The statue could slide back to allow the Computer to reach out with its mechanical arm. The Master Computer and the first generation of Robots were left by their humanoid creators a million years before the 23rd century. The Computer and the Robots originally took commands from their creators, but once they were on their own the Robots learned to repair themselves and create new generations of Robots, and those later generations of Robots answered only to their own kind, not humanoids or the Master Computer.
When the Enterprise discovered the Robots they happened across the Master Computer while exploring the Robots' city. The Computer initially held the crew captive in its mechanical hand; however upon learning of the history of the Computer Spock realised it would take command from humanoids and ordered it to free them. Spock then ordered the Computer to subdue two Robots that were pursuing the party. However the Robots were able to blackmail the crew into releasing them and complying with their demands to give them power rods from the Enterprise or risk the lives of the rest of the Enterprise crew, which the Robots were holding captive. (TOS comic: "UK comic strips, first story arc")