Sergeant Wragge happened to see it there, lying by the side of the road, and decided to take care of it himself. After all, a twelve-inch butcher knife is nothing to be left loose on a public highway. When he noticed those curious stains on the blade, his suspicions were more than aroused and he felt that he must be ready for trouble.The Sergeant's forebodings were swiftly corroborated by the events that followed-robbery, a mysterious disappearance, perhaps murder; so he felt that he was justified in demanding the aid of Scotland Yard. The careful investigations of Inspector Hanslet and Jimmy Waghorn soon had them on the right track; but it was Dr. Priestley's quiet, seemingly enigmatic suggestion that finally unearthed the solution.