An excursion train comes through a tunnel in the Malvern Hills 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 sharp jolt alarms the passengers. The first person to alight is Arthur Heale, the man in charge of the excursion to the delightful spa town of Great Malvern. He walks to the front of the locomotive to see a boulder on the line, but as he is chatting to the driver, fireman and guard, Heale is shot dead by a sniper. Such a puzzling case for the Railway Detective who is despatched from Scotland Yard to find the killer before he can strike again.