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Pip, Bets, Larry and Daisy mistake Ernest "Ern" Goon, the village policeman's nephew, for their good friend Fatty in one of his disguises. They even call him Fatty and Bets gives him a Christmas present meant for Fatty, a notebook with his name on it. When Ern tells all this to Mr. Goon, the policeman asks the children's parents to ban them from solving another mystery. The children then invent a fake mystery about kidnappers and robbers on Christmas Hill, hoping Ern will pass the information on to Mr. Goon. They also encourage Ern to investigate at Christmas Hill, where Larry and Pip plan to flash some lights and Fatty will hide himself to give Ern a scare. Meanwhile, Mr. Goon finds Ern's notes about the "mystery" and threatens to punish him. Mr. Goon then goes out at night to investigate the hill. Ern sets out to follow him but goes in the wrong direction, along the road to "Harry's Folly", where he witnesses some mysterious voices, footsteps and lights. He tells the Find-Outers, who don't believe him at first, but then decide to investigate. Fatty dresses as Ern for fun and consequently Ern is captured and taken to Harry's Folly. Bets suggests to Fatty that Ern may be there, and Fatty investigates, with Pip and Larry in tow. Ern is there and Fatty asks him to wait until the next morning so the crooks could be captured. Inspector Jenks later sends six policecars to Harry's Folly. Ern is rescued and promises to tear up a rude poem that Fatty wrote about Mr. Goon, as both Ern and Mr. Goon think that Ern wrote it.