The Conscience of Marinus was a supercomputer located on the planet Marinus. Originally used as a perfect arbiter and judge, it was later adjusted to control all minds on Marinus to make evil thoughts and actions impossible.
The computer was maintained by Arbitan. It could only be used if all five keys were in place. The Voord, led by Yartek, developed an immunity to its effects, allowing them to exploit the people of Marinus, who had no concept of guile. The machine was deactivated and the keys scattered, for fear that the Voord would get their hands on it and use it to enslave the planet.
Shortly after their arrival on Marinus, the First Doctor and his companions were sent by Arbitan to find and reassemble the keys so that he could reactivate and adjust the machine. They succeeded, but by the time they arrived back, the Voord had already killed Arbitan and taken over his citadel. The use of a forged key in place of one of the real keys caused the machine to explode when the Voord tried to reactivate it, destroying the invaders. The Doctor considered the destruction of the computer to be for the best, thinking morality should be genuine and not imposed by an outside force. (TV: The Keys of Marinus)