This is a mystery book. 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. One cause is the fact that in early youth a miscarriage of justice gave him several years in prison, an experience which cast a stigma on his name and which made it impossible for him, for many years after, to obtain honest employment. But the world is richer, and safer, by Muller's early misfortune. For it was this experience which threw him back on his own peculiar talents for a livelihood, and drove him into the police force. Had he been able to enter any other profession, his genius might have been stunted to a mere pastime, instead of being, as now, utilised for the public good.