On a remote island of glass surrounded by a sea of acid, there is a machine that can remove evil from the minds of an entire population - the Conscience of Marinus. Fearful of its immense power falling into the wrong hands, its sole guardian has scattered the machine's operating keys across the planet.
The TARDIS crew arrive to find the island under attack by the evil Voord. Marinus' last line of defence - and its only hope - is the Conscience machine. The Doctor and his companions must undertake a deadly quest to uncover the Keys of Marinus.