When an epic new Sherlock image like this appears, you know the game is afoot. And while I have to admit, I haven't quite grasped the overall significance of John and Sherlock sitting in their flooded Baker Street flat, after getting out my digital magnifying glass (ie zooming in on PhotoShop) and making one or two creative leaps, I can confirm there are definitely points of interest here.
So what can we deduce from this soaked scenario – and what does it tell us about who might appear in series four of Sherlock?
Moriarty's return? (1, 3)
221B Baker Street wouldn't be complete without Sherlock's violin and the sheet music (1) that accompanies it, but look a bit closer and you'll see that this particular composition is called Miss Me? Now where have we heard that before?
The bone china cup bobbing around in the water is also a reminder of Moriarty – remember the time he sat down to tea with Sherlock in this very room?
At the end of the Christmas special, Sherlock told us he knew exactly what his arch-nemesis was going to do next. He also told us he was dead. So if this isn't a hint that Moriarty will be returning, it's at least a reminder that his shadow will continue to loom large over Sherlock long after he's gone.
Tom Hiddleston as the third Holmes brother? (2)
Ever since Mycroft reminded Sherlock "what happened to the other one" back in series three, fans have been clamouring for Tom Hiddleston to be cast as the third Holmes brother. They've even named him: Sherrinford, the moniker Sir Arthur Conan Doyle had in mind for his detective before he changed it to Sherlock. It was taken up for Holmes number three in a non-canonical story and, most significantly, is the clue word used to reference the third episode by Mark Gatiss and Steven Moffat.
We wouldn't normally expect the Sherlock creators to go with fan casting but back in July Gatiss tweeted an image from Comic-Con of himself and fellow Sherlock star Amanda Abbington with Hiddleston, along with the hashtag #blud – which roughly translates as brother if you know your street talk.
So what's the connection to Hiddleston in this new picture? Well maybe this is a stretch but that worn leather book floating in the water is a copy of William Shakespeare's Henry V. Hiddleston starred as Henry in BBC2's Shakespeare series The Hollow Crown – the very same series that saw Sherlock himself, Benedict Cumberbatch, as Richard III.
Is this a hint that we might see the pair reunited again at some point this series? Or simply a reference to Sherlock Holmes's favourite Shakespeare quote, which appears in the play:
I see you stand like greyhounds in the slips,
Straining upon the start. The game's afoot:
Follow your spirit, and upon this charge
Cry 'God for Harry, England, and Saint George!'
Either way, it sounds like it could be stirring stuff.
A nod to a Doctor Who character (5)
You have to get up pretty close to read her name but the author of this book is Lavinia Smith, a woman who hardcore classic Doctor Who fans will recognise as the loving aunt of one Sarah Jane Smith, who she brought up after her parents were killed in a car crash. Lavinia's not a major Who character – in fact she only appears once in the TV series – but she did raise one of the Doctor's best-loved companions, so she deserves a nod of some sort. And who better to give it to her than Doctor Who writer Mark Gatiss and showrunner Steven Moffat?
Clues to the final episode? (4, 6)
If you thought (2) was a long shot, you'll love this...
See the knife plunged through a stack of papers on Sherlock's mantelpiece? Well his habit of doing just that with his letters is detailed in one of the original Sherlock Holmes stories The Adventure of the Musgrave Ritual, in which Dr Watson recalls that he would often see Holmes's "unanswered correspondence transfixed by a jack-knife into the very centre of his wooden mantelpiece".
In the same passage, Watson notes that his friend "kept his tobacco in the toe end of a Persian slipper". And what do we have here, in exhibit 6, but said Persian slipper.
The Musgrave Ritual – about an ancient family riddle that leads to buried treasure – is one of my favourite of the Sherlock Holmes stories, and I happen to know it's one of Mark Gatiss's too. Since we don't yet have a title or any details of the third and final episode of Sherlock series four, would it be too much to think that these could be hints that this might be it?
Probably, but I'm still holding out hope...
Sherlock series four begins on New Year's Day, 1st January 2017, on BBC1