After a trip to the Scottish Highlands, Cressida Fawcett’s journey back to London on an express train is disrupted by the murder of industrial tycoon Lewis Warriner. Working with Detective Andrews, they interrogate fellow passengers to uncover the murderer. As suspicions mount and another gunshot is heard, Cressida faces the challenge of solving the case with Andrews incapacitated, striving to prevent further deaths before the train reaches its destination.