John Arbinger receives an anonymous note - offering 'protection' from criminal gangs in exchange for ?5,000 - with the impression of a tiny silver horseshoe in the bottom right-hand corner. Ignoring the author's warning about going to the police, Arbinger seeks the help of Superintendent Budd of Scotland Yard. But Budd is too late to save Arbinger from the deadly consequences of his actions, and soon the activities of the Silver Horseshoe threaten the public at large - as well as the lives of Budd and his stalwart companions...