The Introduction sets out the scale of the Victorian investigative revolution and its relation to other literary and journalistic modes such as special correspondence, official commissions, low-life guides, and social exploration. We highlight the prominent role played by female investigators and the local press in developing the new genre as well as key formal features such as the series and the first-person narrative voice. In making the reporter’s own experience central to their narratives, we argue, undercover investigations drew periodical audiences into a new kind of identificatory relationship. Journalists who pretended to be insane, destitute, or otherwise straitened were not merely penetrating closed social spaces but serving as proxies for readers imagining a hypothetical social catastrophe of their own. This impulse to titillate, rather than campaigning zeal, lay behind most undercover forays and underlay the genre’s long-term popularity and, not least, its wide-reaching impact on literary writing of these years.