London, Today: Arthur Drake is only just beginning to understand how he is linked the Invisible Detective's casebook written over eighty years ago. But there's no time to investigate further - a monster is loose in the city, and Arthur must get help from the past... London, 1936: Whatever your problem, the Invisible Detective can find the answer. He is a mystery, an enigma. In fact, the truth about the Invisible Detective is known by only four children - the ones who created him. How can a daring bank robbery be linked to a missing cat? And why are the local rats overrunning houses and attacking people? Only the Invisible Detective can discover the answers as the kids hunt the robbers - and track down a monster...