On demand: "Sherlock: The Final Problem"

Last New Year's Day, Benedict Cumberbatch's Sherlock took his first bow in UK cinemas with a simulcast screening of the roundly enjoyable festive special The Abominable Bride. A year on, and the subtitle of this Holmes' latest investigation The Final Problem suggests it's all over. Where did it go wrong? Online fan chatter informs us that the show's fourth series - unfolding on successive Sundays since this New Year's Day - has, even with the limited three-episode run, been a decidedly mixed bag: one good one, one bad one, and a general feeling that the show has succumbed to that cumbersome mythology that makes for far less lively and engaging viewing than those self-contained, case-specific episodes with which showrunners Steven Moffat and Mark Gatiss first built up this fanbase. Season Four has been labouring towards the revelation that our hero has an unhinged sister, Eurus, who's spent this series running amok in various guises, unrecognised by Sherlock until she took a potshot at Martin Freeman's Watson while impersonating a therapist; the finale found her as the sole inmate of a byzantine offshore asylum, where she holds sway over the staff like a cross between Hannibal Lecter and the puppetmaster in the Saw movies. 

Moffat's intention was clearly to provide Sherlock with a new nemesis, one with easy access to the emotional weakspots that have come to light in the sleuth over three seasons, and Sian Brooke did much to establish Eurus as a credible threat, as masterly in her playing of human beings as she is of the family Stradivarius. Yet it's telling that the episode's pulse only really elevated once Andrew Scott's eternally OTT Moriarty reappeared on the scene, emerging from a helicopter to the strains of Queen's "I Want to Break Free": here, at last, was a rockstar baddie, whose presence, however wayward or malign, had been sorely lacking from all the washed-out asylum business. (For once in recent months, television looked to have been scooped - and long ago - by the movies.) All the fun stuff sat around the edges of this episode: a prologue in which Gatiss's Mycroft finds his enjoyment of a faked-up noir film (prime Gatiss territory) interrupted by an elaborate prank; the revelation that he once played Lady Bracknell in a school production of The Importance of Being Earnest (you can imagine that, and it's funny); a clever misdirect as a figure we initially clock as Cumberbatch-as-Holmes-in-disguise turns out to be nothing of the sort.

And though it came to be obscured by Eurus's very shaggy dog story, The Final Problem did float one genuinely compelling situation - the kind of problem which Moffat might once have spent an entire episode solving - involving a young girl who wakes up aboard an airborne plane on which the other passengers and crew appear to have died. Increasingly, Moffat looks to have been drawn to the fate of characters held in such suspension, faced with the possibility of a nasty crash-and-burn (the events of The Abominable Bride, it transpired, unfolded while Sherlock was dozing on a flight), and - donning our own deerstalkers - we might usefully analyse what this nightmare represents: the influence of Christopher Nolan's Inception, which sent semi-impenetrable vapour trails around the world, on contemporary fantasy-drama? The predicament faced by a showrunner obliged to operate in a holding pattern, and circling endlessly around those fixed points Conan Doyle left behind in the popular imagination? Or merely seepage from all those transatlantic flights the show's creatives have had to take just to get another season together? (Perhaps flights have become to Sherlock what meetings about taxation were to the Star Wars prequels.)

The biggest letdown here came with the revelation this particular scenario was playing out in headspace rather than actual airspace - that it was a figment of an overactive imagination, and no more - which meant the producers didn't have to worry about stumping up to film any dramatic final descent, and Moffat didn't have to explain it, except to explain it away. What these flights of fancy may finally stand for is the show's own trajectory: that as it went stratospheric, picking up viewers overseas, touching down in cinemas worldwide, it became more and more detached from the ground rules Moffat and Gatiss first laid down for themselves - that its once sharp deductive logic, a pleasure crucial to any worthwhile Sherlock variant, turned inexact, if not utterly vague. (One solution in this Problem involved a series of transposed numbers and letters on a gravestone, and was dashed through so offhandedly in the edit as to seem like a gross narrative cheat.)

The sad thing was that enough of the original show's fuselage was still intact for The Final Problem to pass muster as lazy Sunday night viewing, bag of leftover Christmas treats to hand. It's admirable that Moffat continued to give Cumberbatch emotions to play, rather than the bag of tics handed to Robert Downey Jr.'s Sherlock, and it's sweet that the series should play out by affirming Sherlock's friendship to Watson in the face of everything this pair had been through in the course of recent episodes - no matter that it left women looking as secondary to this bromance as they are to the Holmes and Watson of those Guy Ritchie movies. (Mary Watson's closing vision of the pair as "two men sitting in a scruffy flat" only evoked dread memories of Men Behaving Badly.) Rather than sticking the landing - if this is to be the show's final destination - this episode felt more indicative of a downturn, if not a terminal nosedive: as good a point as any for stars and viewers alike to bail out.

Sherlock: The Final Problem is available to watch online here.