Artie Conan Doyle and his best friend Eddie Hamilton, nicknamed Pig, take on a new investigation. Someone stole a giant mechanical dragon from a super-secret and secure warehouse in broad daylight! The creator of the device, Professor Anderson, the Great Magician of the North, is in despair! Young detectives have a difficult job to do: find the stolen dragon and bring the criminal to light. But how to do that? After all, there is not a single clue, and the only evidence is a strange note, which confuses everything even more. None of the adults can help the boys. Artie's dad is too busy at work, and good old Sergeant McCorkle doesn't believe that the dragon was stolen.