Sherlock Holmes stories have been metafictional since there have been Sherlock Holmes stories, what with Holmes complaining that Watson likes to gin up the sensation to maximize his readership. Since the 1970s or so, however, adaptations have taken the more mischievous and/or subversive approach of foregrounding the purported distance between Watson's character "Holmes" and the "real" Holmes, from Billy Wilder's The Private Life of Sherlock Holmes and Michael Dibdin's The Last Sherlock Holmes Story to more recent examples like Michael Chabon's The Final Solution and Mitch Cullin's A Slight Trick of the Mind. The Sherlock special The Abominable Bride is thus yet another excursus into the realm of explicit Holmes metafiction, with the added fillip that it takes on the current series' relationship to the Jeremy Brett Granada adaptation (establishing shots, a few snatches of the opening theme, the discussion of the story title at the end) and to its legions of online critics and fans.
At one level, the episode sent up the Granada series' famous attempts to reconstruct "authentic" period detail, which here becomes a kind of shorthand for Holmes as walking dead, as it were. This was perhaps most obvious in the closing shot, in which the 19th-century 221B set was abruptly juxtaposed with a 21st-century street scene, but also in the repeated references to the Paget illustrations (something for which the Granada series was also known), which here, rather cheekily, are ripped out of their original narrative contexts and made to serve an entirely different purpose. More generally, the undeniably bonkers Gothic plot, which somehow manages to yoke "The Five Orange Pips" (the, er, five orange pips, the revenge plot, and the KKK imagery) to "The Greek Interpreter" (Mycroft, the obliquity of the ecliptic, the avenging woman) to the sort of bizarre church setting one expects from the steampunk Robert Downey, Jr. films, is uncomfortably reminiscent of the later Granada journeys into extended episodes (many of which viewers would like to forget). "Is this silly enough for you yet?" inquires Moriarty. But the repeated breakdowns in both cinematic style (Holmes' second confrontation with Mycroft in particular, with its odd upward angles and lighting) and language, as the 21st century kept erupting into the 19th, reminded viewers that such aspirations to authenticity have a bad habit of pulling apart at the seams when examined too closely. Characters do what the authors want them to do, in good Rosencrantz and Guildenstern are Dead fashion, which is why poor Mrs. Hudson goes from complaining about being reduced to a "plot function" to making everyone tea because, well, that's what she does. Of course, the whole thing is just a cocaine-and-who-knows-what-else dream, and so it's one added layer of irony that Holmes dreams up an "authentic" Holmes based on an adaptation of a series that does not exist in Sherlock's own universe. Speaking of which, much of the episode relies on a stealth pun: shooting guns vs. shooting up. The bride blows her brains out (well, sort of) just as the mysteriously reappearing Moriarty did/does--when she isn't murdering her husband with an awfully phallic rifle--and the solution to her mystery is also the solution to the 21st-century case of revenant Moriarty. Meanwhile, the gun-toting feminist approach to patriarchy, as presented here, is as potentially lethal as Sherlock's own recreational use of cocaine, morphine, and whatever else (and, perhaps, just as addictive?).
The quite deliberately embarrassing "ooh, feminists are the real KKK!" reveal at the end, even when taken as modern-day Sherlock's own drug-addled fantasy about what the women in his life really think*, seems suspiciously like a parody of a certain type of social justice rhetoric. Faced with a room full of avenging angels (well, in KKK garb), Holmes the good ally speechifies at length about silenced women engaging in resistance &c. And it's there that Moriarty wearily pops up to point out how "silly" this all is--both the ridiculous framing (activists as the KKK) and Holmes' purported moment of truth. Here, we gave you what you wanted, the showrunners say to the segment of their audience who complain about the series' sexist aspects; now, isn't it all so ludicrous? (Moriarty's suggestion that Sherlock and John really ought to "elope" is equally sardonic fanservice, yet another shout-out to the sort of fandom ship-teasing that the showrunners have quite calculatedly employed.) Then again, Moriarty's own "defeat" at Reichenbach Falls, where Watson magically shows up to save the day (with yet another gun, possibly Chekhov's), is itself overtly silly, what with the ineffective fistfighting and Moriarty's eventual demise. Nothing much here to be taken seriously.
In terms of how effective all this meta was...well, aside from the more mean-spirited facets of some of it, it might have worked better if so many other authors had not already explored these issues. (A Slight Trick of the Mind and Mr. Holmes are all about why the Sherlock Holmes stories worked; they're also about the ethical limitations of such storytelling.) This perhaps speaks to the source of my ongoing frustration with this series, which is that it keeps imagining that it is more original than it actually is--even when, as here, it is thumbing its nose at people who insist on fetishizing a previous adaptation.
* ETA: A poster on Metafilter makes this interesting argument: "The link for me comes from reading David Graeber’s “Debt: the first 5,000 years” in which he writes about the way societies who based their economies on slave labour had a kind of societal guilt about the institution that expressed itself in (amongst other ways) bloody violence against the slightest hint of slave revolts out of the fear of what such a revolt would do to the slave-owning classes - in other words they feared the worst because they knew deep down that they deserved the worst. By the same argument, if we read the episode as taking part in Sherlock’s head & not representing anything real, then perhaps Moffat’s plot isn’t saying that feminism wants to kill all men, but rather that this is what men feared - that their subjugation of woma[n] meant that they deserved this, even if no woman ever seriously plotted to kill their husbands for some inchoate feminist cause. Perhaps then the feminist plot in this episode really represents Sherlock’s own buried feelings about his treatment of the women in his life, from Hooper to Irene Adler on? Treating women badly seems to be a Sherlock trope & deep down he knows he deserves censure for it."