Does the same apply to Steven Moffat and Mark Gatiss’s contemporary adaptation? Has Sherlock – during its first and second runs, unquestionably the best thing on television – succumbed to success and started buying its own myth? The first instalment of Series 3, ‘The Empty Hearse’, was so infatuated with the brand and its hallmarks – the Belstaff coat, the memory palace, the eternally unconsummated romance between its leading characters – that it forgot to produce a decent story on which to hang them.

Incidentals are entertaining, but ephemeral – the episode was enjoyable, but less for the threadbare, V for Vendetta-meets-The Bruce Partington Plans uptown plot than for the chance to complete the fanwank bingo card (Sherlock and Moriarty snog? Check. John does some earnestly mild swearing? Check. Mrs Hudson makes a homosexual faux pas? Check.) Fun to watch, of course, but a million miles from the complex genius of ‘A Study in Pink’ or ‘A Scandal in Belgravia’.

Thankfully, something genuinely golden shone out amid the slick, smart self-satisfaction: the assured excellence of Benedict Cumberbatch, Martin Freeman and – yes – Amanda Abbington as Mary Morstan, who has so effortlessly turned a double act into a threesome it’s as if she’s been shacked up with the other two for years. Which brings us to ‘The Sign of Three’.

The first hour is a struggle. It’s messy, disjointed and obsessed with being funny. Clever funny. The central premise of the episode – John and Mary getting married – is so overloaded with flashbacks and snippets of other cases that the stream of gags is left to hold it all together.

A lot of the LOLs are admittedly glorious. The stag night bender that goes awry is outstandingly played, climaxing with Sherlock’s deductive faculties failing (identifying a chair as a ‘sitty thing’) in a sea of sick and John going for a high five with no one, while the Great Detective’s play date with young Archie that ends with them looking at maggot-ridden corpses is brilliantly macabre. The call-back to the latter at the wedding is even better – ‘Beheadings!’ the kid enthuses. ‘Lovely little village,’ Sherlock quickly counters, belying his apparent unawareness of social norms – but the overriding sense is that the whole thing is being played for laughs. Sherlock has always been funny, but it used to be a lot more than that.

It isn’t until the final third that some of the old dramatic tension finally makes an appearance, with Sherlock’s Usual Suspects moment of revelation and unmasking of Jonathan Small – the stabby photographer bent on revenge against John Watson’s old CO, Major Sholto. There’s also a very touching ending, with Sherlock establishing that Mary is pregnant at the reception disco.

The warmth and laughter shared by the three main characters is more poignant than any of the overdone schmaltz of the earlier Best Man’s Speech. The sleuth leaving the wedding alone after failing to get a dance with anyone, wrapping himself in the comfort of his old coat, is perhaps the first iconic moment of the series.

Will there be any more? The seafaring Cornishman who so politely disparaged Conan Doyle’s post-Reichenbach Holmes adventures was right, broadly speaking, but he failed to take into account The Hound of the Baskervilles, the greatest Sherlockian adventure of them all – and that was published after the sleuth plunged to his supposed death. Does this mean, therefore, that despite the frothy inconsequentiality of episode one and the rambling hilarity of episode two, this third series of Sherlock still has a classic to come? Christ, I hope so.

Aired at 8.30pm on Sunday 5 January 2014 on BBC One.

> Order Series 3 on DVD on Amazon.

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Watch the Series 3 teaser trailer…

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