With a show as popular - and rarely on - as Sherlock, the internet is always buzzing with speculation about where it will go next when it eventually returns.
One of the biggest reactions to a TV episode in recent years happened after the finale of series two, The Reichenbach Fall, when Sherlock jumped from a rooftop and promptly returned from the dead. For two years, fans came up with endless theories about how the detective faked his death - varying wildly in believability - until series three revealed the actual answer. Such was the depth to which fans dissected the episodes that the explanation in the show actually felt less developed than some of those fans had made up.
And that spirit has carried on in this long gap between series three and four, as fans wonder what will happen next to Sherlock and John, how Mary's pregnancy will turn out and if Moriarty really has returned.
Here are some of the craziest fan theories about Sherlock, those that the detective himself would surely criticise for their huge leaps of logic and questionable deductive ability.
10. Mrs Holmes Is Moriarty
I'm not sure the nuts and bolts of how this would work were considered when this theory was dreamt up, but if Mummy Holmes turned out to be her own son's nemesis it would certainly be one hell of a twist!
This seemingly-random theory does have some basis in the show. Famously, the Napoleon of Crime is a Professor of Mathematics in the original Conan Doyle stories - something that has not been carried over onto Sherlock with Andrew Scott's not-very-professorial Jim Moriarty.
However, in His Last Vow, we learn that Sherlock's mother is a genius mathematician who wrote a book called The Dynamics of Combustion. In the original stories, Moriarty was the one to author a book called The Dynamics of an Asteroid.
Coincidence? Some sections of the fandom don't seem to think so.