“Rat” can only be a reference to “The Giant Rat of Sumatra” which is not a complete, canonical story from Sir Arthur Conan Doyle, but instead merely mentioned by Watson in “The Sussex Vampire.” We know that The Empty House will be the first episode, but they always conflate numerous stories into one.
Episod 1 = The Giant Rat of Sumatra + The Adventure of the Empty House
This one is a bit more confusing than the other hints and could refer to a few stories in which forced or fictional marriages are on the minds of the primary bad guys. In “The Disappearance of Lady Frances Carfax,” Holy Peter woos and marries women in an attempt to steal their fortunes. In “The Solitary Cyclist,” the odious John Woodley attempts a shotgun wedding at the end of the story to steal Violet Smith's fortune. Of the two, we'd prefer “The Solitary Cyclist” as it's an awesome mystery, and Sherlock Holmes beats up someone in a bar. Could have elements of Charles Augustus Milverton where Holmes gets engaged to a maid to get info and then.. leaves…
But “wedding” could also refer to the wedding of John Watson to Mary. Steven Moffat has already discussed waffling back and forth on whether or not he wants this version of Watson to have a go at marriage, and this might be the clue that he's made up his mind. In addition, Moffat has also already stated that John will likely not be spending as much time at Baker Street in the coming season, suggesting that Watson might move out following the Reichenbach fallout. Will John find his Mary? Will Sherlock do everything he can to break them up? Is this going to put our dynamic duo on the outs by the end of the season? Only time will tell.
== The Sign of Four with Mary Morstan ????
== The Disappearance of Lady Frances Carfax??????
== The Solitary Cyclist???
==Charles Augustus Milverton ???
Our guess is that this is a reference to “His Last Bow,” which comes from the Conan Doyle short story collection of the same name. The weird thing about “His Last Bow” is that it is one of the few stories not told from Watson's perspective, instead employing third person narration. This oddity is compounded by the fact that the story itself is less of a mystery and more of a pre-war time espionage story. While this seems out of place among the Holmes canon, the contemporary incarnation already has an abundance of espionage and government conspiracies. Instead of Mycroft leaving Sherlock out of the loop, perhaps this version of “His Last Bow” will see a more James Bond-style storyline.
The weirdest thing about “His Last Bow” is of course that Sherlock retires to become a beekeeper at the end of the story. Would our Benedict Cumberbatch Sherlock really do this? Is this a sign that the show might go on hiatus after season 3?