Once A Study in Scarlet, The Sign of Four and The Hound of the Baskervilles have all been adapted that only leaves one novel length Doyle-Holmes story remaining: The Valley of Fear, a tale of American private eyes and masonic crime rings impacting a more typically Holmesian country house murder. Perhaps the least loved of the novel length stories, it's second (Holmes-less) half in which the back story is told is probably more enjoyable than the initial mystery. But that story of secret societies and violent crime in a mining community in the bleak middle of nowhere could provide ample material for a re-invention. However, it is the framing device that leads Holmes to be interested in the case and eventually wraps it up with a murder he could not prevent, which is perhaps of the most interest to Sherlock fans and future adapters. Holmes' interest is piqued by the arrival of a coded message, only lacking the book to break the code. He must then figure out what book his informant might have and suppose that he too owns in order to crack the code. Something similar appeared in the Sherlock episode The Blind Banker in Series 1, but in this case it leads Holmes to realise the culprit above and beyond the rest of the criminals in the story: Moriarty. Yes, The Valley of Fear is the only story apart from his death in The Final Problem in which Moriarty makes an appearance in Doyle's original canon of Holmes stories and, as such, presents an opportunity to use the TV show's main villain further. He is never seen in person in The Valley of Fear, but it is his actions that threaten the supposed victim and later result in his death. There are a number of ways this could inspire the TV series. Moriarty could make a return from the "grave" just as Sherlock will or, equally, this episode could be set, as the Doyle story written long after The Final Problem, much earlier in the continuity then when it was produced. Or the events of the story could occur as events set in motion and planned by Moriarty before his death. There are plenty of possibilities for Gatiss and Moffat to use the character further even if he remains dead and, if they do, The Valley of Fear should be their starting point.
