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.

William Hartnell
as The Doctor

Carole Ann Ford
as Susan Foreman

Jacqueline Hill
as Barbara Wright

William Russell
as Ian Chesterton

George Coulouris
as Arbitan

Martin Cort
as Voord / Warrior / Ice Soldier / Aydan

Peter Stenson
as Voord / Ice Soldier / Second Judge

Gordon Wales
as Voord

Robin Phillips
as Altos

Katherine Schofield
as Sabetha
CinemaSerf
When the TARDIS materialises on a strange planet, the crew discover that the beach is made of sharp glass and the water of acid. Is something being kept in, or kept out? As they head off to explore, one by one they are captured by the robe-clad "Arbitan" (George Colouris) who explains that he is in ...