The Doctor and his companions land in the city of Arkhaven, the last bastion of civilisation on a doomed world.
The inhabitants of the city are pinning all their hopes on a final desperate gamble for survival. Behind the scenes there are jealous factions at work, secretly contesting for the chance to shape the destiny of a new world. Beneath its ordered surface, Arkhaven is a city of secrets and mysteries where outward appearances can be deceptive.
Is the thing they call the 'Creeper' really at large in Arkhaven's eerie outer zone - and is it beast or machine? What is the hidden force at work that has acted so strangely upon Susan?
With Barbara lost and the countdown to doomsday drawing to a climax, the Doctor must discover the true nature of the final enemy - or is that enemy simply fear itself?