Ian, The Doctor, Barbara and Susan by the Console

The TARDIS arrives on the planet Marinus on an island with a glass beach and surrounded by a sea of acid. On leaving the TARDIS they spot a large pyramid and so The Doctor, Susan, Barbara Wright and Ian Chesterton make there way to it. They are though unaware that they are being followed.

When they arrive at its base they discover the pyramid has many secret entrances. Inside they encounter Arbitan, The 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.

Arbitan explains to them that the society of Marinus is in danger. For hundreds of year, the Conscience was absolute, radiating its power across the planet Marinus, and eliminating all thought of evil. But then the evil Yartek, leader of the alien Voord, worked out how to resist its impulses. They are now seeking to enter the tower and take control of the Conscience.

Arbitan

The Conscience though 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.

Arbitan then asks that The Doctor and his friends help him gather the keys together. It seems that others have tried to accomplish this task but none have returned to the tower. The Doctor refuses Arbitan's request, but when it is realised that they can’t get back to the TARDIS, due to a force field Arbitan places around the ship, The Doctor and his companions are forced into agreeing to retrieve the missing operating keys.

The Voord's Craft

Arbitan gives them wristwatch-like travel dials which will transport them instantaneously to each of the keys' locations in turn. And so the travellers find themselves transported from place to place but in their quest the travellers become separated as they visit many exotic but dangerous locations.

They almost fall prey to the hypnotic influence of disembodied brains in the city of Morphoton. They manage to escape from a ruined citadel which is being overrun by rampaging plant life. They also find themselves in a snowy wasteland where the dangers include wolves, an unscrupulous hunter and a group of fearsome Ice Soldiers. In overcoming each situation they manage to collect three of the four keys as well as finding a fake one.

They finally all meet up again in the city of Millennius but danger still awaits them when Ian is falsely accused of murder and discovers that the legal system is that you are guilty until proven innocent. The Doctor takes on the role of defence counsel and succeeds in postponing the trial to enable him to gather evidence and to work out what really happened. Ian is eventually cleared of all charges and the final key is also located.

The Doctor with Ian and Susan

Having found the four keys, and also one fake, the travellers return to the island. However, they find that in their absence, the Voords have murdered Arbitan and taken control, but Ian Chesterton tricks Yartek into using the fake key along with four of the real ones and, as a result, the Conscience machine is destroyed killing Yartek along with the Voords.

With the force field no longer working The Doctor and his companions flee the burning tower and are able to depart in the TARDIS.