The detective novel is undoubtedly one of the more distinctive literary genres created by modernity. Initiated by Emile Gaboriau, the “roman judiciaire” constitutes a decisive and still understudied stage in the progressive emergence of the detective novel. This essay is devoted to Fortuné du Boisgobey, one of Gaboriau’s heirs and certainly one of the most representative authors of this genre. It studies the ten “judiciaire” novels published by Boisgobey between 1875 and 1889, a period during which the “roman-feuilleton” was the main mode of publication and laid down its constraints, its structures, its figures and its imaginary to the novelists who used it. The essay traces the role of inquiry in Boisgobey’ narratives and shows how its fundamentally hybrid function organized the structure of the text, the system of the characters and of the values depicted.