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
Heron Carvic
as Voice of Morpho (voice)
Edmund Warwick
as Darrius
