Devastating inevitability could be a potent instrument in drama. Has to be robbed of all power just because you know something's coming, that doesn't mean the minute of arrival. Foresight on the audience's part can permit you to play on a sense of menace, a mood of anxiety that is sickening.
For much of its comeback hour, Sherlock does just that – and to great effect. 'The Six Thatchers' builds and it builds, as our title character is haunted by nightmares of inexorable darkness, just as Mary Watson's past catches up with her.
It's an attempt to throw us off guard, but it is a twist that robs the story of much of its impetus – as though Steven Moffat and Mark Gatiss second-guessed themselves, and fatally complex things along the way.
It is a real pity that 'The Six Thatchers' doesn't stick the landing because there's a lot to like here.
On the first watch, it feels as though Gatiss is over-egging the minute taker's 'little old lady' character in the opening scene because it's all a part of the subterfuge, but this funny bit works in retrospect.
The same can be said for the frothy tone of the entire first act. But that's completely the point. It is all a clever ploy to lull us into a false awareness of security.
Afterwards, once the laughs begin to become more infrequent and the walls close in tighter and tighter on our heroes, what follows is a tense and involving 40-or so minutes. That suffocating darkness comes back into play, and it's milked for all its worth.
Performances from the regulars are not uniformly weak, even as they're each requested to step beyond their comfort zones. Benedict Cumberbatch presents us with a Sherlock who is more delicate and less assured than ever before; Martin Freeman gets to play John Watson at both his most and least sympathetic, and Amanda Abbington is clearly relishing the chance to explore so many different sides of Mary, from adoring mum to unstoppable hitwoman.
In a guest turn as Mary's unhinged ex-coworker, Sacha Dhawan also does terrific work, while Rachel Talalay's dynamic direction means that when the episode goes for the activity jugular – as with Sherlock and AJ's swimming pool struggle – it matches the best of modern Bond.
But ends are not unimportant. So significant. And for all its powerful part components, 'The Six Thatchers' doesn't quite come together in the closing act. Instead, it fizzles to some finish – and not simply because all that Moriarty teasing has so far come to nought.
The thinking behind the exit scene of Mary seems clear - given that she is spent the entire episode running in the vengeful AJ, having the killer blow be hit from another direction must've looked like the smart action to take on paper.
Moreover, her ending being brought about not by her sins, but by Sherlock's conceit and egotism, appears a suitably canny although unkind device - especially as her self-sacrifice evens the score for her previous attempt on Sherlock's life.
But in its attempts subvert our expectations and to not be unpredictable, 'The Six Thatchers' ironically ends up delivering something much more hackneyed - a character we understand is doomed jumping before a bullet to truly save our lead that is vital.
For every one of the thought and preparation that must've gone into this most critical of pictures, the departure as it eventually plays out of Mary feels like old hat.
Despite the best efforts of the performers – Freeman is particularly superb – this awareness of finished acquaintance renders the whole thing rather dead. Mary's departure itself may be inevitable, but did its performance need not to be quite unpredictable?
Going past that, though, the episode does mine that is later some rich drama from her death. It leads to the largest ever division between Sherlock and John – and unlike the last time, Holmes was seen to betray his friend, this time there's no Mary to mend the rift. Her absence is the rift.
And while, for the greater good, Sherlock faking his death was for all his pompous grandstanding, this time he is got no such defence. His tongue-lashing might've been motivated by love for a pal, but Mary is still cost her life by his inability to control his conceit. That is a huge betrayal, especially given his vow to secure the Watsons at all costs.
Sherlock's done erroneous – and he knows it. Who believed we'd ever see this man, so dedicated to his persona of self-confidence and self-assurance, so openly acknowledge that he's in turmoil? We've seen chinks in his armour before, but usually merely in the presence of his most-trusted friends and allies. But to turn to some stranger for guidance and help, Sherlock must be in dire straits indeed.
John, meanwhile, must cope with his guilt – not only for neglecting to protect his wife but for having run an emotional affair. Mary's tasked Sherlock but is Holmes even in a fit state right now to save himself?
It's an enticing internet that Moffat and Gatiss have left for themselves to untangle, so while it is not perfect, 'The Six Thatchers' does effectively establish a new playing field for the amazing game, one where nothing is sure, and no one is not dangerous.
This isn't Sherlock at its greatest, but where we end up is fulfilling enough to ensure you'll be tuning in again – for 'The Lying Detective' – to see where the story goes next.