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From: http://www.bbc.co.uk/doctorwho/classic/episodeguide

The TARDIS arrives on the planet Marinus on an island of glass surrounded by a sea of acid. The travellers are forced by the elderly Arbitan to retrieve four of the five operating keys to a machine called the Conscience of Marinus, of which he is the keeper. These have been hidden in different locations around the planet to prevent them falling into the hands of the evil Yartek and his Voord warriors, who plan to seize the machine and use its originally benevolent mind-influencing power for their own sinister purposes.

Now the machine has been modified to overcome the Voords and can be reactivated, so the keys must be recovered. In their quest, the travellers - transported from place to place by Arbitan's wristwatch-like travel dials - have adventures in the city of Morphoton; in a building besieged by ambulatory plants; with a lecherous and murderous trapper; and in the city of Millennius where Ian is falsely accused of murder and discovers that the legal rule is 'guilty until proven innocent'.

The keys are eventually retrieved and the travellers return to the island. Arbitan has been killed by Yartek, who apparently tricks Ian into handing over the final key. Ian, however, passes a fake key instead and when Yartek tries to use it the machine explodes, killing him and the Voords.