The Conscience of Marinus was a supercomputer located on the planet Marinus, capable of controlling all minds on the planet. Originally used as a perfect arbiter and judge because it was strictly impartial, the Conscience was later adjusted gradually over the years to make evil thoughts and actions impossible. It became the envy of everyone because all crime was eradicated.
The computer was maintained by Arbitan. It could only be used if all five keys were in place. The Voords, 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 Voords would get their hands on it and use it to enslave the planet. (TV: The Keys of Marinus) In the years that followed, the Keepers of the Conscience had managed to reverse the effect of the immunity, such that its reassembly would allow its use against Yartek once more. (PROSE: Doctor Who and the Keys of Marinus)
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 Voords had already killed Arbitan and taken over his citadel. The use of a fake key in place of one of the real keys caused the machine to explode when the Voords 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)