Ern, the nephew of Inspector Goon, asks the Five Find-Outers to find a mystery for him to solve. Fatty tells him about an imaginary, dark case on top of a hill. Uncle and nephew compete to see who will be the first to solve the non-existent puzzle, but Ern, unwittingly, becomes the reason for a real mystery to be uncovered. The Five Find-Outers will find themselves in the heart of a desolate forest, in an isolated mansion, where a dangerous, buried secret awaits them…