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.
| Actor | Role |
|---|---|
 William HartnellThe Doctor | The Doctor |
 Carole Ann FordSusan Foreman | Susan Foreman |
 Jacqueline HillBarbara Wright | Barbara Wright |
 William RussellIan Chesterton | Ian Chesterton |
 George CoulourisArbitan | Arbitan |
| MMartin CortVoord / Warrior / Ice Soldier / Aydan | Voord / Warrior / Ice Soldier / Aydan |
| PPeter StensonVoord / Ice Soldier / Second Judge | Voord / Ice Soldier / Second Judge |
| GGordon WalesVoord | Voord |
 Robin PhillipsAltos | Altos |
| KKatherine SchofieldSabetha | Sabetha |
| HHeron CarvicVoice of Morpho (voice) | Voice of Morpho (voice) |
 Edmund WarwickDarrius | Darrius |
 Francis de WolffVasor | Vasor |
| MMichael AllabyIce Soldier / Larn | Ice Soldier / Larn |
| AAlan JamesIce Soldier / First Judge / Guard | Ice Soldier / First Judge / Guard |
 Anthony VernerIce Soldier | Ice Soldier |
| HHenley ThomasTarron | Tarron |
 Raf De La TorreSenior Judge | Senior Judge |
| FFiona WalkerKala | Kala |
 Donald PickeringEyesen | Eyesen |
| SStephen DartnellYartek | Yartek |
| DDougie DeanEprin (uncredited) | Eprin (uncredited) |