On 21st century telly however, the greatest departure Sherlock has ever taken from Conan Doyle’s canon looks to be that marriage matters. And if anyone’s changing it isn’t the newly betrothed Watson but the ineligible bachelor himself, Sherlock.

The best episodes of Sherlock are those that push The Great Detective past the boundaries of being a consulting automaton in a fantastic coat and into the uncharted realm of being a human being, and in that respect no episode has ever achieved more than what ‘Three’ does.

If ‘The Empty Hearse’ raised up Sherlock Holmes as an indestructible ideal, a meta-fictional legend, then ‘Three’ reminds us that there is a man of flesh and blood and – most improbably – heart, beneath the iconic deerstalker.

It feels redundant to praise Cumberbatch and Freeman, but they excel themselves as a comic double act. Cumberbatch especially so, as Sherlock is pushed into the world of wedding planning and considering feelings, culimating in a second act that shows him at his most human and as we’ve never seen him before. It brings out a hitherto unseen (and highly GIF-able) comic magic that you never expected from either actor or character.

Sadly we can’t divulge any more than that. In fact, you’ll be able to judge how good ‘The Sign of Three’ is simply by the veritable wedding registry of unmentionable but ever so exciting plot points the BBC have forbade previewers from talking about. It makes sense. You don’t want the big day spoiled.

And what a big day it is, for Sherlock more than his best friend or Mary (the increasingly enchanting Amanda Abbington), with a Best Man’s speech that is not only touching, hilarious, and dramatic, but acts as the nexus of the episode: a clever structuring of story that will likely be even more impressive on a second and third viewing. And you’ll want to watch it again just to see the joyous parade of never-before-seen cases that are sure to stoke the imaginations of fan-fiction writers everywhere.

Moffat, Gatiss and Thompson’s script is an ensemble work of beauty. A sign of three talents at their best. Mercifully straightforward despite flitting through time, it’s pinpoint sharp in all the right places and isn’t afraid to have fun in others. And as if allowing the writing time to leave an impact, the over-eager hyper-stylised editing of ‘The Empty Hearse’ is mercifully toned down, a little more ‘Sherlock circa 2012′. It’s as inventive as ever, but you can watch safe in the knowledge that you won’t come away feeling like you’ve experienced laser eye surgery.

Do be prepared for your eyes have moistened several times before the end however. Because Mrs Hudson might be right. But if Sherlock is changing then it’s only for the better.

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

> Order Series 3 on DVD on Amazon.

> Buy the Series 1-2 boxset on Amazon.

Watch the Series 3 teaser trailer…

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