MARINUS a remote force-shielded island set in a sea of acid, governed by THE CONSCIENCE the ultimate computer which rules and balances the gentle life of Marinus, guarded by ARBITAN THE KEEPER ruthless protector of a peace-loving race threatened by YARTEK Warlord of the brutal sub-human Voords, sworn enemy of Arbitan and of Marinus, who has within his grasp THE KEYS OF MARINUS the Conscience's vital micro-circuits, the doors of good and evil. Can the Doctor find the hidden circuits in time? Arbitan's command was 'Find them, OR DIE!'