Seymour Merriman, the junior partner in a firm of London wine merchants, is traveling by motorcycle from Avignon to Bordeaux when his vacation grinds to a sudden halt. Out of gas, he follows a passing truck to a mill where an English firm manufactures pit-prop supports for coal mines. He's given two liters of gas and is about to depart when he notices something peculiar — the truck he followed was marked No. 4, but now it's labelled No. 3. His curiosity piqued, Seymour makes some inquiries that draw him and his friend, Claud Hilliard, further into intrigue. The amateur sleuths are certain that the Pit-Prop Syndicate is a front for some kind of illegal activity, and when a member of the Syndicate is found murdered in a London taxi, the mystery becomes a case for the professionals of Scotland Yard.

A tense tale of smuggling, romance, and murder, this novel combines intricate plotting with episodes of mechanical ingenuity. Its 1920s setting, ranging from the South of France to Northern England, abounds in period atmosphere. Author Freeman Wills Crofts, a leader of the Golden Age of British crime fiction, was acclaimed by The New York Times Book Review as "the all-time master of the intricate timetable alibi" and by Raymond Chandler as "the soundest builder of them all."