In a weird jungle valley, the Victorian explorer Rupert Von Thal saves Bloomsbury novelist Beatrice Mapp from a ghastly death in the grip of a monstrous mantis. But this is no Lost World of the dinosaurs. According to their travelling companions, the Doctor and Nyssa, all four have been transported back to a primitive Earth that should never have existed!

Further down the valley is the vast city where the scorpions live. Walking, talking, intelligent scorpions, ruled over by their cruel and sinister master. The Doctor and Nyssa are being drawn ever tighter into the clutches of... the boy that time forgot.