The second episode of Series 3, written by Stephen Thompson who penned the last series' finale The Reichenbach Fall, is titled The Sign of Three. This would appear to point to Doyle's novel The Sign of Four as the episode's inspiration. This shouldn't come as a surprise as The Sign of Four is the second Holmes story, with the first, A Study in Scarlet, and third, A Scandal in Bohemia, already being adapted as the opening episodes of the first two series. It is also the second of just four full-length Holmes novels along with the already adapted A Study in Scarlet and The Hound of the Baskervilles. The Sign of Four is perhaps most noteworthy in the wider Holmes canon for introducing Mary Morstan, who would become Watson's wife. It seems likely that some version of Mary will appear in this season of Sherlock so that element at least is likely to be part of the adaptation. Another of the more memorable elements of the story, Holmes' elaborate and impressive deductions based on Watson's pocket watch, have already been adapted into the show as Holmes' deductions around Watson's phone in A Study in Pink. To what extent we are likely to see the rest of The Sign of Four's plot in The Sign of Three is as yet unclear. Drawn specifically from the history of the 1857 Indian Rebellion and telling a plot involving villainous Sikhs and a midget with a blow-dart, it's unlikely that the topicality or racial elements are going to survive an adaptation. However, the concept of a pact between a group of convicts and the promise of hidden treasure that may come to Mary will probably play some part in the case.
