On a small island with a glass beach, surrounded by an acid sea, on the planet Marinus, stands a tower with many secret entrances. Within the tower is Arbitan, Keeper of the Conscience of Marinus, a vast computer developed two millennia earlier as a vast justice machine which kept law and order across the entire planet. For seven hundred years, the Conscience was absolute, radiating its power across the planet Marinus, and eliminating all thought of evil. But then Yartek, leader of the alien Voord, worked out how to resist its impulses.
When The Doctor and his companions Barbara Wright, Ian Chesterton and Susan arrive on the island, they are brought into the tower to an audience with Arbitan, who explains that the society of Marinus is in danger. Several submersibles containing Voord, humanoid creatures protected by amphibian-like black rubber wet suits, have washed up on the beach. Inspired by Yartek, the Voord are seeking to enter the tower and take control of the Conscience.
Arbitan explains that the Conscience has now been upgraded sufficiently to control the Voord again, but needs to be activated. Years earlier Arbitan had prevented the Conscience from falling into Voord control by separating the five Keys needed to regulate it. The five keys are in different locations - one is in Arbitan's possession, but the other four are scattered over Marinus. The keys can only be found by following directions pre-set into travel dials, watch-like devices with the power to transport the wearer across the planet to the correct locations. Arbitan asks that the Doctor and his friends help him fend off the Voord by gathering the keys together. Others have tried to accomplish this task - even Arbitan's own daughter - but none have returned to the tower.