Arthur Conan Doyle’s works featuring the brilliant Sherlock Holmes are regarded today as some of the greatest detective fiction ever written. It must be kept in mind, however, that before Conan Doyle, “detective fiction” was not a specific genre. Edgar Allan Poe had made significant progress in popularizing this type of story with his detective Auguste Dupin some sixty years before Sherlock Holmes emerged, and Conan Doyle was far from shy about his admiration for Poe. Conan Doyle did not only write Sherlock Holmes books, however. Decades before, he had begun writing Gothic tales.
In regard to The Sign of Four, therefore, it is appropriate to consider it both a detective story and a Gothic tale. The central case that occupies Holmes—solving the mystery that Mary has brought to him—puts it squarely within the detective fiction genre. Elements that link it to Gothic romance include the character of Mary, an innocent damsel in need of aid; the mysterious settings of London’s dark streets, especially the gloomy, castle-like Pondicherry Lodge; the search for lost treasure; and the exotic, Orientalist elements of the Indian islands.