Travelling third-class on a crowded train from Yorkshire to London, after purchasing a necklace from a shady knight of the realm, Hatton Garden diamond merchant, David Hyde, can't believe his good fortune. More passengers filter into his compartment before the train departs, a heavy thunderstorm raging all around them. The train rushes on with a rattle and a roar, clattering into the Highpen Tunnel and, when it emerges, David Hyde is slumped in his seat, shot through the head, the necklace missing. Local police, Inspector Parker and Police Constable Brent, undertake initial enquiries but the complexities of the case later require Scotland Yard Detective Inspector Kilby to take the lead. There are many questions to be answered in this puzzling case.