When BBC Sherlock aired their Christmas special "The Abominable Bride" in December 2015, fans were asking questions and analyzing it until the release of series 3 a year later. Some are still analyzing it.
I noticed something when I was going through screencaps of the episode a couple of days later. When I was picking out caps of Victorian Holmes and Watson’s carriage ride post-first plane scene, I found one that stuck out to me: Because for half of a second, Watson was not Watson. Watson was actually John.
The scene starts out with Holmes and Watson on their way to find Mary, and Watson is upset because Holmes knows where Mary is but won’t tell him yet.
This is a few scenes after the very first non-Mind Palace plane scene (after Watson gets mad at Holmes for taking drugs and they leave Baker Street) but Holmes still gets Watson mixed up with John in his head for a brief moment, when Watson says “Sherlock, tell me where my bloody wife is, you pompous prick, or I’ll punch your lights out!”
Why the sudden switch up? Aside from John calling him “Sherlock” instead of “Holmes,” the line sounds like something Watson would say, not John. And for all we know, Sherlock could just be imagining John using his first name because he has so recently slipped from actual modern-day consciousness (when John and Sherlock use first names) back to his Mind Palace. And in the very next line, the MP John is “back to normal” and replaced with Watson, who says, “Holmes, where is she?”
Holmes doesn't get modern John and Victorian Watson swapped much in his head during particularly lucid moments like this one. His Mind Palace is too secure for that. And in this scene, Holmes is acting much more aware than he was at the flat when the drugs still had quite a hold on him. They seem to have worn off better by this time. And in this scene they’re outside of London, so they’ve been in the carriage for a little while—presumably giving Holmes time to gather his thoughts.
This shirt (that Holmes’ MP John is wearing) is not the same shirt non-MP John is wearing during the plane scene. You would think, if Sherlock was getting the two Johns confused because he’s just “awoken” from his Mind Palace only to dive back in, that the modern John who pops up in his Victorian Mind Palace sequence would be wearing the same thing as the John he has just seen in real life. But for some reason, that's not the case here.
The two shirts are really similar, and at first I thought they only looked different because of the lighting, but I’ve looked through pictures from the "His Last Vow" tarmac scene and each modern scene from "The Abominable Bride" and non-MP John’s shirt is definitely more blue than Holmes’ split-second John swap.
So I got to thinking—have we seen this shirt before? Why would Sherlock's Mind Palace conjure John in a brand new shirt? So I went digging through old scenes. I knew this shirt was probably from S3 (because let’s be honest, it’s WAY too stylish for S1 and S2 John), so I started with "His Last Vow" and made my way backwards until I came to the "The Sign of Three" stag night scene.
Just so you can see the change in lighting:
The jackets are different than the one he is wearing in TAB (MP John’s is his black leather one), and it’s too dark to see if MP John is wearing the sweater that goes with the stag night outfit. But it’s 100% the same dress shirt, the most important and apparent article of clothing in both scenes.
So let’s revisit what’s going on in the TAB Mind Palace scene. Watson is anxious about Mary, whom Holmes has just said is “in danger.” They’re setting out to find her. Holmes knows Mary’s whereabouts, but he hasn’t yet told Watson, so Watson is grilling him about where she is. That’s when Holmes says, “Oh, good old Watson, how would we fill the time if you didn’t ask questions?”
So we have three themes so far: Mary, idle time, and questions—each of which was a facet of John’s stag night. Sherlock and John went out drinking as a last hurrah before John married Mary, they got drunk too soon into the night and had a lot of time to wait around, so they played a question game.
After modern John switches back to Victorian Watson, Holmes finally answers his question about Mary’s location, the church. But then he says, “She thinks she’s found the solution and for no better reason than that, she has put herself in the path of considerable danger. What an excellent choice of wife.”
Does that description sound familiar at all? Whether he realizes it before it’s out of his mouth or not, Holmes isn’t describing Mary; he is describing himself. An excellent choice of wife indeed.
In tagging this scene, in which Sherlock’s Mind Palace parallels himself to John’s spouse, with the stag night, which is known throughout the fandom as Sherlock’s “last chance,” Moffat and Gatiss are setting Sherlock up as the one who should have married John. The Victorian carriage ride is a re-working of John's modern stag night. The only other way they could have tied this scene to the wedding is to put John in his groom’s tux, which is a little heavy-handed.
But besides, John in his tux sets him up with Mary, while Sherlock and John have their culminating moment on the stag night. This callback to "The Sign of Three" via wardrobe adds to the unending list of meta of Sherlock as “the other groom”—and makes apparent that it was not just a cute wink-nudge to put Sherlock in a tux matching John’s. It was a sign of the recurring theme of Sherlock as John’s fated partner, not Mary.