Before Sherlock season 4 came to air, showrunners Steven Moffat and Mark Gatiss issued the usual three clues pertaining to each episode: Thatcher, Smith and Sherrinford. The first episode was called "The Six Thatchers," and we already know that the focus of episode 2 is a villain played by Toby Jones, named Culverton Smith. So, it follows that Sherrinford will likely be a focal point of this season's final episode, "The Final Problem."

Google the name, and you’ll immediately discover that it’s the name of the third Holmes brother- sort of. In fact, Sherrinford Holmes was the name that Arthur Conan Doyle was going to give to his fictional detective, but he settled on Sherlock instead. Though Mycroft features in Doyle’s work as Sherlock’s brother, there is no mention of Sherrinford, or any other siblings. Instead, William S. Baring-Gould wrote a fictional biography of Sherlock, and it was he who introduced the idea of a third Holmes brother, called Sherrinford.

The idea made sense. At the time in which the Sherlock Holmes stories were written, the eldest Holmes son have would been required to stay at home and manage the estate, while one of the favored professions for a second son was to become a civil servant - a job which falls to Mycroft (and leaves Sherlock to his detective work). As well as his inclusion by Baring, Sherrinford Holmes has cropped up in other Sherlock Holmes literary works, and is often said to have intellectual capabilities to rival even that of Mycroft and Sherlock.

Sherlock has alluded to there being another sibling before; at the end of season 3, Mycroft told Sherlock “I’m not given to outbursts of brotherly compassion. You know what happened to the other one.” That kicked off fan speculation, which was then further fueled by the "Sherrinford" clue. What seemingly cemented the idea of this mysterious Holmes sibling, was Mycroft making a call at the end of "The Six Thatchers," and asking to be put through to Sherrinford.

This being Sherlock, it’s unlikely that Sherrinford will appear in the way or form we expect. A popular fandom desire is to see the character being played by Tom Hiddleston, and Gatiss cheerfully stoked that fire by posing for a picture at San Diego Comic-Con with Hiddleston and Amanda Abbington, which he uploaded with the caption "Blud." While that is a popular slang term for a friend considered to be as close as a brother, Gatiss probably knew exactly the reaction that picture would invoke, and the speculation that would follow. It could be that the Sherlock fandom’s wish will be granted, and Hiddleston will guest as the eldest Holmes brother - but let’s face it, Moffat and Gatiss love to keep their fans surprised.

Sherrinford might be a sister rather than a brother, for a start, or they might not be related in any way to the Holmes family. They might not even appear at all - at least, not in any recognizable form. Remember how Sherlock seems to have recollection of a dog named Redbeard? Mycroft has spoken of him before, and revealed that Sherlock used to write stories in which he and the dog were pirates. Something happened with Redbeard, and we are still none the wiser. It could be that Sherrinford is tied into some traumatic childhood memory including Redbeard that Sherlock needs to work through. Maybe that is his final problem.

It's more likely, though, that "The Final Problem" has something to do with Moriarty. He is, after all, Sherlock’s arch nemesis. Dead or not, that man knows how to play a game, and it could be that Sherrinford is needed to help in some way with bringing Moriarty's legacy down once and for all (either that, or the pair were in cahoots with one another). Given that Mycroft and Sherlock are hardly the most compassionate of people, there is a distinct possibility that Sherrinford Holmes might be on the opposite side of the game.

Questions still remain following "The Six Thatchers." Was Vivian Norbury really acting alone, or was there a connection to someone like Moriarty, Culverton Smith, or Sherrinford Homes? Who is ‘E’, the mysterious bus stop lady? It’s hard to believe that John would be the type to embark upon an affair while his wife has just given birth to their baby, and we also don’t believe that we’ve seen the last of E, either. Is she deliberately trying to get close to John because she’s working for Moriarty or Sherrinford? Is she Sherrinford?

The theory that Sherrinford will swoop in and save the day for John and Sherlock is by far the most preferable (though not necessarily the most likely) of these theories, but there is one (admittedly far-fetched) theory that this could all end in the darkest of tragedies. Sherrinford Holmes appears in a Doctor Who novel called All-Consuming Fire, which features Sherlock and John on an adventure with the Doctor. In the story, Sherrinford is a member of a cult, and Sherlock ends up shooting him in order to save John’s life.

Now, the Doctor is not going to appear in Sherlock, but Moffat is the showrunner on Doctor Who, and Gatiss also writes for it. They both share the most obscure knowledge surrounding both Doctor Who and Sherlock Holmes, and frequently drop in random references to Doyle’s works during Sherlock episodes. Given that time and again we’ve been told this is the darkest season of Sherlock yet, and the fact that John and Sherlock’s relationship is under strain after the death of Mary, perhaps this terrible choice could be Sherlock’s Final Problem.

Then again, of course, any and every Sherrinford reference could be a complete red herring! Hopefully, the question of ‘Who is Sherrinford Holmes?’ will soon be answered once and for all.

Sherlock continues with "The Lying Detective" on January 8th on BBC 1 and Masterpiece