Third in the mystery series involving the five ""Find-Outers"" and their dog, this is more or less an English equivalent of the Bobbsey Twins mystery story, and equally harmless. ""Idea man"" Fatty, Pip, Bets, Larry, and Daisy, ages ranging from eight to twelve, stumble on a ready made mystery when they discover an oddly furnished room in an empty house. After some skirmishes with the blundering ""Clear-Orf"", the village policeman, in which the children experiment with disguises and invisible ink to tease him, their determination to discover the mystery of the secret room leads them into inquiries, explorations, a capture and escape and finally to the seizure of an international gang of thieves when their good friend inspector Jenks steps in. Bright British chatter, but the plot is a bit too thin and hurried. Not top drawer Blyton.