Sherlock review: a TV show that seems keen to avoid its final destination

In one of Arthur Conan Doyle's own favourite Sherlock Holmes stories, The Case of the Red-Headed League, the mercurial detective takes one glance at an ordinary-looking fellow and deduces only the following: he is a former labourer, active snuff-taker and current Freemason, who has spent time in China and done a lot of writing. The visitor and Dr Watson are stunned. Holmes talks them dryly through his processes (of the guy's tattoo, Holmes "has contributed to the literature of the subject"). "Well, I never!" laughs the visitor. "I thought at first you had done something clever, but I see there was nothing in it after all."

To Sherlockologists, rarely mistreated by Steven Moffat and Mark Gatiss’s cheerily propulsive adaptation (Sunday, BBC One, 8.30pm), hearing those words repeated with a small concession to modernity (“It’s dead simple, innit?”) has a double-braced pleasure: lightly demonstrating the writers’ expertise in the source material, while scattering clues both mysterious and literary throughout the narrative. Under the magnifying glass of superfans, what seems like nothing at all turns out to have done something very clever.

In the first episode of what may be the programme's last series (it concludes with an episode called The Final Problem, and aligning the busy schedules of its stars Benedict Cumberbatch and Martin Freeman isn't getting any easier), Holmes doesn't even bother with the gent's case. Summarily delivered from the tangles at the end of season three, which we watched a full three years ago, he now dismisses a stack of them (and as many book references) as too trivial to be worth his time. You suspect that Moffatt and Gatiss (who wrote the first episode, The Six Thatchers, alone) are also discarding surplus ideas, as though in a fire sale.

Yet The Six Thatchers still seems to amass text – based on The Six Napoleons, while stitched through with a deathly premonition, The Appointment in Samarra, borrowed from Somerset Maughm – as though overwrought with research. In passing, we get one locked-room mystery (the discovery of a long-dead body incinerated in an apparently empty car) but otherwise, stretching from London to Tbilisi, Iraq to Morocco, and back again, the plot goes all about the houses.

Through it all, everyone expects superhuman ability from Holmes, a self-described “high-functioning sociopath”, while complaining that he can’t be more human. Cumberbatch’s Holmes, a tousle-haired, mercurial automaton, is still a career best, and Freeman’s Watson, now a much more suave family man with semi-retired superspy Mary (Amanda Abbington), is less his adoring, blogger biographer than peevish doormat. Partly, this is because television writers require emotional development from long-running shows, but also, as the wittily integrated images of video calls, blinking GPS maps, and evaporating sheaves of text suggest, these days everyone with a smartphone is a potential Holmes. “What are you, Wikipedia?” Holmes asks his brother Mycroft, a pleasingly supercilious Gatiss. “Yes,” he responds, presumably contributing to the literature on the subject. Simple, innit?

That the episode feels like a shaggy dog story, albeit immensely well paced and filmed by director Rachel Talalay, hints at a certain anxiety of television and the detective story, as does its denouement, designed to up the emotional stakes and encourage character development. The words belong to the villain, when finally revealed, channelling the logic of a shark, but they apply just as well to these spinners of mystery, old and new: “We have to keep moving, or we die.”