Parts: 6
Writer: Terry Nation
Airdates: 11 April 1964 - 16 May 1964
Production Code: E

Episode Titles: The Sea of Death; The Velvet Web; The Screaming Jungle; The Snows of Terror; Sentence of Death; The Keys of Marinus

Doctor: first
Companions: Susan, Ian, Barbara

Creature Feature: Voords, Brains of Morphoton

Story: The TARDIS materializes on a glass beach of the planet Marinus, but Susan's anticipations of a good paddling session are dashed when the wading pools turn out to be not water, but acid. Further explorations turn up several beached glass submarines and a large building, which the TARDIS crew start to explore.

Inside the building, they meet Arbitan, an older gentleman who maintains the Conscience of Marinus, a large machine that once kept the population of Marinus nonviolent by controlling their thoughts and actions. The machine is only operable when all five keys are present, but Arbitan only possesses one of the keys. Worried that the evil Voords are intending to take over, and knowing what would happen if the Machine fell into their hands, Arbitan asks for the Doctor's help in recovering the other four keys. As insurance that the Doctor will give his aid, Arbitan has rendered the TARDIS inaccessible. With no other option, the Doctor and his three companions take travel-dial bracelets and set off on their quest.

The search for the other four keys will take the Doctor, Susan, Ian and Barbara all over the entire planet of Marinus. They must safely traverse a land where paradise hides an ugly truth, a minefield of traps, a ravenous whispering jungle, an icy wilderness and a formal accusation of murder against one of their party. And even if they can recover all four keys, their struggles may not be over.