In Émile Gaboriau’s 1868 Monsieur Lecoq, the titular detective turns to contemporary cartographic techniques for his investigation of a triple murder at the outskirts of Second Empire Paris. This essay analyses the novel’s reproduced crime scene map in order to propose the terrain vague as a foundational site in the spatial imaginary of the detective novel genre. Functioning historically as a liminal area for urban renewal and symbolically as a blank page onto which violence gets inscribed, Gaboriau’s terrain vague registers a tension between criminal violence and the desire for cartographic, rational, and judicial mastery. It both reveals an enduring generic ambivalence between history and form and sets the stage for twentieth-century banlieue crime fictions.