1865. A passenger train stands in York station. Jack Follis, the guard, patrols the platform to make sure that everyone is safely aboard. He returns to the brake van to load a box into it. Follis is alarmed by a smell of burning. Before he can find out the cause, there is an explosion and the whole van is engulfed in flames.

In response to a summons from the North Eastern Railway, Robert Colbeck and Victor Leeming are sent to investigate. Leeming is not convinced that a crime has taken place, but Colbeck disagrees. Although the information they received was scant, he is convinced that it is a murder case.
The longer the investigation goes on, the more complex it becomes. Guilt shifts to and fro at a bewildering speed. It takes the combined skills of the detectives to identify and catch the person responsible for causing Jack Follis's death.