Sherlock: The Six Thatchers - a spoiler-free preview

Sherlock's fourth season debuts on New Year's Day with an episode titled the Six Thatchers – and a sneak peek reveals it's set to put a smile on viewers' faces.

Journalists were invited to a special screening of the show ahead of its national broadcast, but were urged to avoid sharing spoilers by both the BBC and the show's creators, Mark Gatiss and Steven Moffat.

So what follows is a general overview of what fans can expect - without specific plot details.

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The Six Thatchers is loosely based on Sir Arthur Conan Doyle's The Adventure of the Six Napoleons, in which Sherlock Holmes and Dr Watson investigate a series of attacks on plaster busts of Napoleon.

Paul Jones in the Radio Times says it is better than the last time-travelling Sherlock, The Abominable Bride, but it's not the "the best Sherlock ever". That, he claims, is The Reichenbach Fall from the second series.

The Six Thatchers fails to have as many clever twists and turns or "the hypnotically intense relationship between Sherlock and Moriarty at its core", he says, but it's still got "enough to keep both general viewers and fan boys/girls engaged".

The episode features new parents Watson and wife Mary doing "endearingly mundane baby stuff", Uncle Sherlock on babysitting duty and even a comic moment with a dog. There's also plenty of action, including a "gritty Bond-style fight scene" for Benedict Cumberbatch's detective hero.

We also learn more about Mary's backstory as a special operative and find out something "shocking" about Watson, continues Jones, before warning fans: "Don't get your hopes up for another return for Moriarty."

Louisa Mellor on Den of Geek says Gatiss gets things moving "at a brisk, even hyperactive, pace" and, after a very moody scene, "piles gag on top of gag".

Freeman and Cumberbatch can do action and heart-break, she adds, "but these two never seem happier than when they're landing punchlines" - and Gatiss gives them plenty.

However, "all the lightness is paid off with a turn towards the dark" in the final third of the episode, says Mellor.

Ultimately, she says, the episode is reminiscent of the Shakespeare in Love quote that what the public really wants is "comedy, love and a bit with a dog".

The Six Thatchers "has all that and more", she teases.

Sherlock airs on BBC1 at 8.30pm on 1 January. The Lying Detective follows on 8 January and a yet-to-be-named series finale will be shown on 15 January.