It seems impossible to begin anywhere other than the ending. As I have noted before, this is a script that blatantly advocates for the extra-judicial murder of Rupert Murdoch. Sure, yes, Magnussen is only a transparent metaphor for Murdoch and not Murdoch himself, but all the same, and especially given how willing Moffat has been in interviews to double down and say that he thinks killing Magnusson was the right thing to do, it's hard to overemphasize the moment, especially given the glorious bluntness with which Mary puts it: "People like Magnussen should be killed. That’s why there are people like me."
And indeed, this quote gets at one of the central questions of His Last Vow, namely "what exactly sort of person is Mary Watson?" Actually, this is in some ways the only central question of His Last Vow. Certainly a central question is not the superficial issue of "how far ahead of the game is Sherlock," although this is possibly worth unpacking. We have, by this point, been trained by two consecutive episodes to realize that this is not actually a question upon which Sherlock is inclined to put much weight. The nature of the game is deliberately constructed to twist and wriggle around. Much of the episode is structured around a pair of contrived editing tricks, and while there are occasional clues ("I have an excellent memory') and the episode does technically play fair, it's still blatantly changing the rules of the episode in arbitrary and essentially unguessable ways.
Whether you think this is clever or not is largely a personal decision. But in what we might call the normal order f things, the point of these fair but unguessable twists would be to find ways of putting the hero in considerable danger. And yet in His Last Vow, the two times in which Sherlock is disastrously wrong (as opposed to when he's just blindsided by Mary) are not actually particular problems for him. When he's wrong about the glasses in the restaurant it's essentially irrelevant - he moves calmly on to his Christmas plan barely skipping a beat. Being wrong about Appledore's physical existence is at least more of a problem, but it's clear he always had "shoot the fucker in the head" as a fallback plan, what with telling John to bring his gun.
Which makes sense. Sherlock, after all, is an ontological character, defined as the one who is always ahead of everybody else in the game. And so the question in His Last Vow is never really whether Sherlock is going to win. The question is what winning is going to end up meaning - a question that's foregrounded from the moment we learn that the case requires him to give into his addictions, which is to say, from the first time we see him in the episode.
It is in this context that the episode's real question, the nature of Mary Watson, must be understood. Even though it's not really even raised as a question until the halfway point (Mary drops out of the story after the scene at the hospital, and doesn't resurface until she shoots Sherlock), the entire episode is about her. And we know this from the title, which is set up explicitly in the previous episode. But the title further emphasizes that this is a story about the consequences and nature of victory. This isn't a story about who Mary is - and to be fair, the answer to that is neither terribly interesting nor terribly surprising. Sure, her shooting Sherlock is surprising, but once that happens, the revelation that the person who shot Sherlock is an assassin is not. Nor is it a story about who Magnussen is. It's a story about a vow, and about the meaning and consequences thereof.
This puts us in familiar territory with Moffat, not least because of the fate of Mary's textual equivalent. The entirety of Sherlock Season Three is built around the way in which Mary is a narrative time bomb. In a standard narrative, the nature of the bomb's explosion would be Mary's death, so that John can take blood-stained and suitably grim revenge. Especially since she's pregnant, which is worth, like, double points when fridging a character. And so the constant tension in this story - which is, of course, just Moffat's standard "what sort of story are we telling here" tension - becomes a constant threat that something is going to go terribly wrong for Mary.
It doesn't, of course. And that is in many ways the point. Sherlock's last vow could never really go unfulfilled. It would go against the nature of him as a hero, at least in Moffat's conception of what that means. This isn't about being perfect, clearly - indeed, Sherlock gets almost every single call wrong in this story. It's about something altogether subtler - something that goes back to Mary's line, and also to Mycroft's observation that Sherlock fancies himself a dragonslayer. Heroes exist, for Moffat, for the purposes of going to extremes that we cannot.
This is, ultimately, the real content of the ending. Sherlock is tragically wrong when he proclaims himself to not be a hero. Because what are our heroes for if not to save us from bullying monstrosities like Magnussen? Does this in its own way make Sherlock monstrous? Of course it does, to an extent that genuinely terrifies him, hence the shot in which we see him as a child in the face of the SWAT team and helicopters. But nevertheless, it is heroic. Sherlock saves not just John and Mary's marriage, but everyone Magnussen owned.
But His Last Vow is not some grim meditation on the monstrosity of heroes. This is an aspect of it, certainly, but the idea that heroes are just the monsters we like is a premise, not the point of the exercise. For all the sense that Sherlock has crossed some sort of line by putting a bullet in Magnussen, he gets off scot free at the end of the story, if only via a timely intervention by Moriarty. This is not a story about the angst of the hero. Nor does it ever seem like one, or else the addiction plot thread would have played out very differently.
Because, of course, the dramatic heart of the episode is the scene among Sherlock, John, Mary, and Mrs. Hudson. It's an astonishing scene in which everybody puts in a jaw-dropping performance. (Really, watch it and look at how much Amanda Abbington contributes to the scene despite getting exactly one word of dialogue - a word she delivers with astonishing nuance.) It's a scene in which line after line is stellar, and almost every subsequent scene exists entirely to unpick the consequences of it.
Of course it's great, though. There is perhaps no plot more Moffaty than "Sherlock resolves a marital dispute that erupts when John's wife turns out to be a top class assassin." But this is because it addresses the theme that's been consuming Moffat for nearly a decade now: how do you make a "realistic" psychology for a hero that doesn't devolve into a deconstructionist rejection of the basic idea of heroism. And so we have John and Mary, desperately trying to balance the fact that they are heroes, with all the madness that entails, and people, and Sherlock killing dragons just to keep them together.
Holding it all together is Nick Hurran, upping his game once again. As ever, his willingness to embrace the artifice of television serves Moffat's script well. Hurran never lets go of the fact that this is a story, adding beautiful touches of sheer artificiality. (My favorite is the plant that moves across the room as Sherlock falls, which happens for no reason but to make it look like the room is actually tilting, despite the fact that the story is in no way trying to suggest that it is, although the use of Christmas lights to smooth a transition from Baker Street to Sherlocks' parents is also gorgeously bonkers.) Obviously the "Sherlock figures out how not to die" sequence is particularly good in his hands, and it is in hindsight something of a wonder that it took until Hurran's sixth episode under Moffat for anyone to give him a proper, honest to god dream sequence. But he's also very, very sharp in the small character moments, with an impeccable sense of how to use closeups and reaction shots.
The result is an episode that is as experimental and postmodern as anything that Moffat has done, but that nevertheless feels oddly grounded and straightforward. Moffat has a growing sense of when to just hand solid drama to skilled actors and get out of their way. After a year of Doctor Who where he seemed intent on accelerating the pace more and more, here he really starts to explore the benefits of slamming on the brakes and lingering on a scene. Having learned how much he can get away with trimming, here is where he really starts to show how to balance that, starting with as breakneck a pace as he's ever managed, and ending with a methodical resolution that takes vast and deserved amounts of time to focus on the Watsons. The result is simply one of the best ninety minutes of television ever produced - an unabashed masterpiece made all the better once you realize that this isn't just a good day at the office, but the triumphant debut of a new style for Moffat, and the point where he goes from an innovative and experimental writer to one who has learned a tremendous amount from his experiments and has moved on to learning exactly when to deploy those tricks.