Everything we know about Sherlock series 4

Dust off your deerstalker. Pop your Belstaff collar. The BBC has revealed Sherlock will return to our screens on New Years Day.

So, now the game's afoot, come with us as we pore over the teaser trailers for The Six Thatchers and The Lying Detective - the titles for the first two of the four-part series  - to see just what we'll be getting when Benedict Cumberbatch reprises his role as Baker Street's most eminent resident.

Right then. What would Cumberbatch/Sherlock do? Foremost, contort your Otter-like visage into an exacting squint, folding one arm across your chest and leaving your other hand free to pluck clues from the very air. Then get a grip and realise, as is the custom of the BBC's Sherlock, these titles are drawing on the original works of Sir Arthur Conan Doyle, so best just give them a read.

The Six Thatchers links to The Adventure Of The Six Napoleons

What happens in the book? Doyle's The Adventure of the Six Napoleons (which you can read here) sees Lestrade bring Sherlock a case concerning the systematic stealing and destruction of busts of French Emperor Napoleon. Lestrade believes they're hunting for a Gallic antagoniser, but Holmes identifies a more pertinent link: all the busts are from the same mould, meaning the vandal doesn't have it in for Napoleon, they're just looking for something contained inside this specific series of busts. 

There follows a murder, further bust smashing, and a hunt for the final ornament - which happens to contain a valuable stolen pearl, hidden there to be reclaimed by the original thief. 

What could happen in the TV show? Forget pearls: in the BBC's contemporary setting, the most valuable commodity that always has Sherlock et al. scurrying about is information. Information that invariably leads to important people getting into trouble, or bad people gaining access to scary things like nuclear launch codes or international secrets. 

The Victorian special The Abominable Bride concluded with Sherlock apparently working out that Moriarty was indeed dead, but had left others to carry out his plans for him. We'd presume that in this first episode, Sherlock and Moriarty's stooge are hunting for a key piece of information that Moriarty has left behind in order for his work to continue. This information is hidden in something that relates to the late Margaret Thatcher: either an institution she helped establish, or a place that carries a statue or physical representation of her legacy. It won't be about Thatcher or a hatred of her legacy, but rather as with Doyle's original work, will hang on something of value being hidden in a place associated with the former Prime Minister.

This piece of information could hinge on that one word that Sherlock wrote in his diary during concluding moments of The Abominable Bride: 'Redbeard'.

As to who Moriarty's stooge will be? Well, we already know...

The Lying Detective / The Adventure of the Dying Detective

What happens in the book? Holmes appears to contract a deadly disease, for which Dr Watson demands to treat him. Refusing to allow himself to be treated, Holmes instructs Watson to seek out a man by the name of Culverton Smith, to meet with him privately at 221B Baker Street. Watson finds Smith, explains that the infamous Sherlock Holmes wishes to meet with him, and departs ahead of the man in order to hide in Baker Street himself and watch the meeting unfold.

It transpires that Smith had sent Sherlock a trick ivory box with a sharp spring trap, stabbing the opener and infecting them with a deadly illness. Smith explains that this illness killed his own nephew. Sherlock then reveals that he didn't in fact contract the illness, but lied about his health in order to bring Smith out of hiding to his home. Watson emerges as a witness to the confession and Smith is arrested. 

What could happen in the TV show? Toby Jones (above) was revealed by Steven Moffat to be one of the main villains of series four, announcing that he would "[bring] to life one of Doyle's finest villains." Jones has also been confirmed to not appear in the first episode of the new series but will make his first appearance in episode two, The Lying Detective. The teaser for the series has also already hinted at someone being unwell, with Watson positioned over their sickbed. 

We're thinking that Sherlock will fake a terminal illness in order to bring Toby Jones' character - Culverton Smith - out of hiding. Smith, continuing the work of Moriarty with information he's gained in the previous episode, may believe that Sherlock is truly close to death and thus expose himself in order to prevent the twisted games from coming to an early halt. Why bother carrying out mastermind criminal plans if there's no mastermind to unpick them?

Sherlock will convince everyone around him that he's really close to death - even John Watson - before revealing his double bluff. Check out the season four trailer for hints on Sherlock's illness - from the nightmarish visions of nurses to his greying, bearded complexion. 

Once Sherlock has established what Smith is up to, it will then be a race across the final two episodes of the series to prevent him from fulfilling Moriarty's final plot - which we're thinking revolves around Sherrinford...

"Who?" 

Sherrinford was teased by the Sherlock bunch at this year's San Diego Comic-Con - a name that crops up in a fictional biography by Sherlock scholar William Stuart Baring-Gould, Sherlock Holmes of Baker Street.

In Baring-Gould's work, Sherrinford is the eldest of the three Holmes brothers, who looks after the Holmes estate while Mycroft and Sherlock concern themselves with running the nation and solving crimes.

The name Sherrinford has an older history still, appearing in the notes of Sir Arthur Conan Doyle as a name he had considered for his detective hero before settling on Sherlock. The 'third Holmes' brother has been hinted to in the BBC's adaptations before, when Mycroft name-dropped "the other one"...

If a hunt for 'Thatcher' leads to 'Smith', we're thinking that information regarding Sherrinford, the third Holmes brother that's never spoken of, will become central to the concluding episodes of this fourth season.

Perhaps Sherlock killed his eldest brother, and Smith will ruin his reputation before carrying out Moriarty's final plans? Maybe Smith is Sherrinford, whom everyone had thought was dead and gone? Or possibly, just possibly, we've been watching too much Sherlock and got ahead of ourselves.

Still, it's fun to speculate, right?