In the market for a race horse, wealthy, beautiful Isabel Herne settled herself in a Cambridge hotel to be near the Newmarket sales. She had hoped to find a filly. Instead she found a promising young man of great charm. He promised that his name was Edward Gower, and he promised that he, too, was in the market for a filly. Indeed, he knew where an excellent one could be bought on the hush-hush for a good price. And if the flattered and attracted Isabel would give him her check for half the money, he would put up the other half. And then he promised they would go to Yorkshire to meet his titled parents. And then they might even be married. And then he was gone. And so was Isabel's cashed check. And so was the horse.

Isabel takes her sad story to her friend the redoubtable Ludovic Travers, head of London's renowned Broad Street Detective Agency. Travers soon finds out that what seemed to be a simple, albeit heartless, case of swindle is merely one layer in a jumbo sandwich of crime-a many-splendored affair that numbers among its chewy charms blackmail, black magic, a black sheep, and murder. And, with customary gusto, he soon reduces it to crumbs after the perilous pursuit of an archcriminal that draws him from the foggy streets of London to a deadly confrontation on a lonely Riviera beach.