December 1864. As a cold winter wind scours the Worcestershire countryside, an excursion train comes through a tunnel in the Malvern Hills, only to be confronted by a blockage on the line ahead. The driver manages to slow the train down so that the impact is minimised, but the passengers are alarmed. The first person to alight is Cyril Hubbleday, the man in charge of the excursion. He walks to the front of the locomotive - and, as he is talking to the driver, fireman and guard, he is shot dead by a sniper. With Christmas around the corner, Inspector Robert Colbeck and Sergeant Victor Leeming are under pressure to solve the case quickly. However, between enemies in the shadows and an investigation hampered by heavy snow, their hunt for a cold-blooded killer is far from straightforward.