A year to the day since Sherlock tackled his last mystery The Abominable Bride (inside his own head) he's back, kicking off the long-awaited series four with a new adventure entitled The Six Thatchers.

Here's all you need to know about the return of Sherlock Holmes...

What time and date is Sherlock series 4 episode 1 on?

As is now traditional, the new series of Sherlock begins on New Year's Day, that's 1st January 2017 to be precise.

The exact time is yet to be confirmed by BBC1 but if we had to guess, we'd say a show of this magnitude will get a 9pm berth.

Who’s starring in it?

Benedict Cumberbatch as Sherlock Holmes. Of course. Martin Freeman as John Watson. Of course. Amanda Abbington as Mary and Mark Gatiss as Mycroft, defo. Una Stubbs as Mrs Hudson, almost certainly. But after that it all gets a bit vague. We know Mr Selfridge's Sacha Dawan is among the guest-stars, as a character named AJ, but we don't know for sure which episode he's in.

What's The Six Thatchers about?

The opening episode is based on Sir Arthur Conan Doyle's Sherlock Holmes story The Adventure of the Six Napoleons, in which a mysterious thief steals and destroys a series of busts of the French Emperor. But in this case, Napoleon becomes former British PM Margaret Thatcher and the shadow of the late Moriarty looms large over proceedings.

Here's the official synopsis...

“Sherlock waits to see where Moriarty will make his posthumous move. One mysterious case in particular baffles Scotland Yard – but Sherlock is more interested in a seemingly trivial detail. Why is someone destroying images of the late Prime Minister Margaret Thatcher? Is there a madman on the loose? Or is there a much darker purpose at work? Something with its roots deep in Mary Watson's past…”

There's actually an entry in John Watson’s real-life blog (run by the BBC) called The Six Thatchers. It lays out one of John and Sherlock’s cases and it does seem to be at least superficially similar to the episode. Read it here

What else is going to happen?

A new character is joining the cast. We don't know his or her name yet but it ends with Watson...

Yes, we'll meet John and Mary's newborn baby. As will Sherlock, who – as we can see from the picture above – is likely to find the whole situation rather puzzling (maybe it's time someone sat him down and told him about the birds and the bees). 

We also have good reason to believe that the new episode will see Sherlock on the trail of his arch-nemesis Moriarty – even if he is dead…

At the end of the last series, Sherlock resurfaced from a drug-induced reverie proclaiming "Moriarty is dead. More importantly, I know exactly what he's going to do next…"

So it seems Sherlock has used the case of The Abominable Bride – in which the culprits turn out to be a group of women banding together to carry out the work of a fallen comrade – to help him understand Moriarty’s apparent return from the dead. It's therefore possible we'll be meeting more of Moriarty’s associates, even if we won’t see Andrew Scott’s villain in person again.

Although with Sherlock, anything is possible…

Sherlock series 4: The Six Thatchers is on New Year's Day, 1st January 2017, on BBC1