Joseph Muller, Secret Service detective of the Imperial Austrian police, is one of the great experts in his profession. In personality he differs greatly from other famous detectives. He has neither the impressive authority of Sherlock Holmes, nor the keen brilliancy of Monsieur Lecoq. Muller is a small, slight, plain-looking man, of indefinite age, and of much humbleness of mien. A naturally retiring, modest disposition, and two external causes are the reasons for Muller's humbleness of manner, which is his chief characteristic.