By the late 19th century, a new phenomenon emerged: tourism. Along with it, large hotels appeared, which targeted an exclusively wealthy clientele. This concentrated luxury in shared spaces quickly attracted a new category of thieves, the "mice" of the hotels. Despite this flattering nickname, they were high-class criminals: although coming from working-class backgrounds, they were educated, multilingual, and well-presented. They rented a room, spent the following nights robbing their neighbors, and then left as if nothing had happened. They are never violent and try to remain as inconspicuous as possible.

Zorg Ostrovski is one of them. Born in Ukraine, a Russian citizen, he operated for about forty years, especially in Germany, where he was known as the "Black Phantom of Kaiserhof," but also in England, Italy, and mainly France. Known to European law enforcement and once wanted by the notorious "Tigers' Brigades," he remained remarkably discreet, to the point of never appearing in the criminal record. However, he profoundly and continually influenced 20th-century popular culture and was the anonymous inspiration for Arsène Lupin and the Phantom.

To give him substance, the French author Patrice Lajoye traced the faint clues left behind by this true chameleon: police reports, interrogation reports, letters, and, most notably, an extensive archive from the press of that era.