It seemed an innocent enough puzzle at first, a truck with one number plate when first seen, but another when seen a few hours later. But Seymour Merriman and his friend, Claud Hilliard, soon become convinced that the Pit- Prop Syndicate is a front for some sort of illegal activity. They just can't figure out the what or how. But when one of the members of the syndicate is found murdered in a taxi, it becomes a matter for the professional detectives of Scotland Yard who must solve the secret of The Pit-Prop Syndicate