The Keys of Marinus

The Keys Of Marinus by Terry Nation, directed by John Gorrie

On the planet Marinus, the scientist Arbitan locks the travellers out of the TARDIS in a desperate bid to convince them to attempt a quest to find the lost keys which power the Conscience of Marinus, a powerful computer which is Marinus' last hope against the onslaught of Yartek and his evil Voords.