There is a certain majesty in simplicity, Alexander Pope once said; and after the tortuously-tangled (albeit extremely enjoyable) A Scandal in Belgravia, it’s a breath of fresh, Devonian air to find that second episode in this series of Sherlock is far less complex. In fact, it’s a pretty straightforward thriller about chemical warfare, cover-ups and a colossal canine. It’s also suspenseful, spooky and superb.

We were more worried about this story being updated than any other (like Lawrence Camley, the fictional novelist interviewed by Alan Partridge in the first Knowing Me, Knowing You, the original novel is an all-time favourite) but our fears were entirely groundless.

It’s partly Withnail & I meets Chimera, a funny trip to the countryside mingled with ominous laboratories and something monstrous running amok, but it’s also unmistakably the heir to the Baskerville legend.

Mark Gatiss ensures all the iconic images are in place – most notably the man on the tor: Sherlock stood on a windswept outcrop surveying the desolate Dartmoor landscape is a timelessly wonderful image – but in an expertly-crafted modern context, familiar names such as Lyons, Barrymore, Frankland and Stapleton all appearing in new and unexpected ways.

Henry Knight (a far better gag than the more scrupulously accurate ‘Henry Baronet’) calls into 221B with a story of his father’s death at the slavering jaws of a gigantic hound – a creature which menaced him during a visit to the moorland where his old boy met his demise.

Holmes, Watson and later Lestrade (who embraces the palpable holiday air of the trip more than most: ‘Nice to get London out of your lungs’) travel to the South West to find a military research base, a fog that won’t do the inspector’s respiratory systems any good and a spectral dog that isn’t quite the hellish horror that anyone’s expecting.

Benedict Cumberbatch’s performance in the title role astonishes yet again – the Minority Report-ish scene of Sherlock searching through the ‘out-of-the-way knowledge’ in his mind palace, finding and discarding Elvis and ‘The Liberty Bell’ in the process before locating the information he needs, is a masterpiece of physicality; the fireside scenes of him admitting fear and doubt are genuinely unsettling; and his ecstatic passive smoking when Henry Knight sparks up at Baker Street, chasing the second-hand fumes like a cat after a dangling piece of string, is hilarious. He deserves plaudits for the way he keeps stretching the role to new dimensions, but it’s high time his co-star received his share of the acclaim.

All too often, Martin Freeman has been dismissed as some kind of one-trick pony, good at playing the solidly wry and amiable good guy but little else. Here, he shines like a rabbit injected with jellyfish genes – particularly when John Watson believes himself to be trapped in a deserted laboratory with the monster they’re searching for. ‘Jesus Christ!’ he shouts to his associate in the shrill, wavering voice of bewildered terror. ‘It was the hound!’

Elsewhere, his subtle demonstrations of hurt at Sherlock’s rebuff, ‘I don’t have any friends’, charm when dining with Dr Louise Mortimer, and unease at being in the midst of something more strange and inexplicable than anything he’s encountered previously (the look on his face when he first sees Dartmoor sums up in a single expression what Conan Doyle describes as ‘the grim suggestiveness of the barren waste’) lay bare what a fine and underrated actor he is.

Of the guest cast, Russell Tovey gives it some serious emoting as Henry Knight, wailing through his repressed memories and H.O.U.N.D. induced-hallucinations without ever really convincing; Gordon Kennedy is better as gay gastropublican Gary, although his choice of partner is as unlikely as his choice of pet – no wonder Sherlock is baffled by sentiment – while Clive Mantle is best of all as the gregarious scientist-cum-murderous, toxic fog-producing villain Dr Frankland – mostly because he seems to be simultaneously the most obvious and least likely suspect.

Fans of the original story and lovers of rabbits may well have been hoodwinked, as CultBox was, by Amelia Bullmore’s mildly menacing Dr Stapleton and missed the vital clue in Frankland’s reference to a cell rather than mobile phone number. (It’s a moment of genius because sounds it out-of-place immediately but as nobody else draws attention to it, one supposes it has no relevance. The Great Detective, thankfully, never assumes anything.)

Finally, there’s the mutant mongrel itself. It isn’t anything of the sort, of course – just an evil-looking mutt hired by Gary and Billy to drum up business for their boozer – and happily, like all the previous hounds that have haunted Holmes in film and TV for nearly a hundred years, it looks gloriously rubbish when it finally appears.

Having a convincingly chilling cur has always been beside the point in The Hound of the Baskervilles – it’s all about the suggestively scary glimpses through the mist and the melancholy howls drifting across the moor – and in a version where the beast’s wrongdoings are in people’s poisoned minds, it’s more important than ever that it looks just the right side of ridiculous.

Young Frankland in a gas mask, the caged monkeys, and everyone’s delirious imaginings are far more frightening than any four-legged fiend. Even the doggers (‘Oh, Mr Selden, you’ve done it again!’) cause deeper distress.

There is, however one other disturbing thing about The Hounds of Baskerville. It’s conclusion, with Holmes off to see a man about a dog, means that there’s only one more episode of the series left – and the closing shots of a cell with the sleuth’s name scraped into a mirror by a manic-looking Moriarty suggest that the fall at Reichenbach will be as long and unpleasant for Sherlock as the empty months after its transmission will be for viewers.

Aired at 8.30pm on Sunday 8th January 2012 on BBC One.

> Order the Series 2 DVD on Amazon.

> Buy the Series 1 DVD on Amazon.

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