Under a lake, in the dripping gloom of an underwater base, stands a gleaming black space ship, recovered from the lake bed. Nothing is inside - but when the base crew start dying, they make a terrible discovery: ghosts are real! And their friends are refusing to stay dead! The Doctor and Clara arrive to find a base under siege from beyond the grave. But how can the dead be walking? What has brought them back? When the Doctor discovers the truth, it is more terrifying than any simple ghost story.