An early adventure for everyone's favourite time traveller. When the TARDIS lands on the planet Marinus, the Doctor (William Hartnell), Ian, Barbara and Susan are captured by Arbitan, Keeper of the Conscience of Marinus. The Conscience is an all-powerful machine which controls the planet in an attempt to regulate crime, but it requires four missing crystals to complete its function. Otherwise, the machine could fall into the hands of the Voord, led by the evil Yartek. The TARDIS crew are thus charged by Arbitan with recovering the crystals before it is too late.