A sequel to The Clue of the Black Cat (1965) involving Bobby Thiriet and his friends and Commissioner Sinet the qua non of the Parisian police force. When a mule is knocked down on the expressway, he has obviously been earlier used by some crooks while the youngsters are determined to keep him from the knackers. Literally putting a cart before the mule, they are led to a valuable substance called tetrital. . . . . Well, it's kind of a put up story even though Mr. Berna never talks it down to his readers who will again enjoy the lively interest of the proceedings and the precision of the Parisian surroundings.