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.
as The Doctor
as Susan Foreman
as Barbara Wright
as Ian Chesterton
as Arbitan
as Voord / Warrior / Ice Soldier / Aydan
as Voord / Ice Soldier / Second Judge
as Voord
as Altos
as Sabetha
as Voice of Morpho (voice)
as Darrius
as Vasor
as Ice Soldier / Larn
as Ice Soldier / First Judge / Guard
as Ice Soldier
as Tarron
as Senior Judge
as Kala
as Eyesen
as Yartek
as Eprin (uncredited)
