So we now know the title of Sherlock series four's concluding episode, and it sounds very… well... final. 

What is the eponymous Final Problem Sherlock will be facing on 15th January? And is it really as conclusive as it sounds? 

As Sherlock Holmes fans will know, The Final Problem is the name of the original Sir Arthur Conan Doyle short story in which Holmes apparently meets his end, plunging from the top of the Reichenbach Falls in the clutches of his arch enemy Moriarty. 

Of course, we now know that the detective wasn’t killed after all – and we also know that Benedict Cumberbatch's version of the fall has been done, back at the end of series two. 

So the Final problem that we see in January is at least unlikely to include that part.

One of the other things that is referenced in Doyle’s Final Problem is the rounding up and capture of Moriarty’s extensive gang, including his various lieutenants and right-hand men.

In last year’s Sherlock special, The Abominable Bride, we were told that Moriarty really is dead, despite the surprise appearance of his face on TV screens around the country at the end of series three. But it’s also made clear that his influence will continue for some time from beyond the grave.

"Moriarty is dead,” says Sherlock. “More importantly, I know exactly what he's going to do next." And the recent synopsis for series four opener The Six Thatchers reiterates that, with Sherlock waiting for Moriarty to make his “posthumous move”. 

That means some of Moriarty’s former associates are likely to be involved in whatever he has planned, so perhaps the finality of the Final Problem will be that Sherlock solves Moriarty’s last puzzle, tracks them all down and brings his criminal empire to an end once and for all.

If that’s the case, it suggests a story arc that will run throughout the three episodes (and, if we’re lucky, some appearances from Andrew Scott in flashbacks and so on).

Of course, there could be a more final finality to that Final Problem. Could it be that this is exactly what it sounds like – after a nice round ten episodes, the very last adventure for Sherlock and John?
    
“It might be the end of an era. It feels like the end of an era, to be honest," the increasingly busy Benedict Cumberbatch told GQ in September.

He later clarified that he did not mean that Sherlock was over, but the language both he and co-creator Steven Moffat have used around the conclusion to the series make it clear that something pretty seismic is going to happen which will make it difficult to return to Sherlock and John any time soon.

"It goes to a place where it will be pretty hard to follow on immediately," said Cumberbatch, while Moffat told AP "You’ll have to see the fourth season to realise why, for now, [new series are] not going to happen again at the same regularity that it has been happening."

So not a definite end to Sherlock but certainly a major event that means it will at least be some time until we see more of it (and in Sherlock time, some time means a very long time indeed).

So what could this major event be? It would have to be catastrophic enough to massively disrupt Sherlock and John's lives but – given that a comeback is not being ruled out – not so big that it would end their partnership for ever.

Only one thing really seems to fit the bill, and that's the death of John's wife Mary.

There is a precedent for this. In the original Sherlock Holmes stories, Dr Watson's wife dies at some point between the end of The Final Problem and Sherlock’s return from the dead. It's not dramatised, but only mentioned in passing.

In Sherlock, though, Mary is much more integral to the plot and, along with her backstory as a former assassin and international woman of mystery, that pretty much ensures that her death would be anything but quiet. It would also send shockwaves through Sherlock and John's lives – and if it was related to one of their cases, it might even manage to destroy their friendship.

Amanda Abbington, who plays Mary, has already hinted at a departure for her character, posting a tweet in July that referred to her "golden wrap" on Sherlock. No amount of backtracking is going to convince me that she meant anything other than that she had filmed her final episode of Sherlock. She's an actor and she knows perfectly well what golden wrap means.

Of course, leaving the show doesn't necessarily mean dying – Abbington's tweet also featured a photo of Mary's discarded wedding ring, so perhaps something happens to drive a wedge between her and John. But given what they've already been through and survived – she shot his best friend and systematically lied about her dark past –plus the fact that they've just had a child together, that seems unlikely. Which could mean the most permanent of exits for Mary.

Having said all this, it's quite possible that something from the minds of Mark Gatiss and Steven Moffat (rather than from Sir Arthur Conan Doyle) will provide another explanation for why the show needs to take a major sabbatical. And it could be that Abbington used the phrase "golden wrap" simply because there is no new Sherlock anywhere on the horizon, if ever. 

One way or another, on Sunday 15th January we'll have an answer to this particular problem – even if it isn't completely final...

Sherlock series four: The Six Thatchers airs at 8:30pm on New Year's Day 2017 on BBC1, with episode two, The Lying Detective, on Sunday 8th January and finale The Final Problem on Sunday 15th January