It's Mary. And the look of absolute horror and shock on Sherlock's face is only matched by me screaming in surprise. Because Sherlock Holmes, the man who can deduce anything about anyone, can't see past the end of his nose when it comes to the people he loves. And god help him, he does love Mary. He cares for her because she is John's pressure point and he knows it. He knows that John loosing Mary would absolutely break his best friend and therefore if John cares about Mary then so will Sherlock. Sherlock never even considered that Mary could be a two timing agent. She dazzled all of us. In the first episode of this season, I remarked that there was something suspicious about Mary but didn't really give it much thought. In the second episode, I was so totally enamored of Mary and her desire to not break up JohnLock that any doubt I had about her was washed away. She was a member of the family now. She belonged with John and Sherlock and that's all there was to that. Again, that's the mark of good TV writing. That I could, in the span of one episode, forget the slight oddness that was Mary's original appearance (knowing skip code for instance) just because she made the two main characters so happy only to have it turned upside down in the very next episode is startling. And then it gets worse because Mary shoots Sherlock and all the feelings and emotions came crashing down.
There is some timey wimey stuff here (this IS a Moffat episode after all) but I want to take this in linear order. Back at Baker Street, John is enraged. As he has every right to be. John can't figure out what he did to deserve this--of all the women in the world, why did he fall in love with a psychopath who used to kill people for a living? Let's talk about John Watson for a moment. John is NOT and never has been the average man some adaptations make him out to be. He is not as smart or deductive as Sherlock, but he's not the everyman. John is an addict of a different sort. But it's not drugs. It's danger. John Watson needs, craves, demands a life of danger. This is something he tries very hard to deny. What John wants and what he needs do not align. He wants the life in the suburbs with a doctor's wife and to live as everyone else. But that's not what he needs. What he needs is the thrill of the chase, the blood pounding in his ears, the constant imminent threat of death. As Sherlock says, "you're a doctor who went off to war, a married man who couldn't live in the suburbs for more than month before you went into a drug house of junkies and a man with a best friend who solves crimes as an alternative to getting high. Even our landlady was part of a drug cartel!" So of course, subconsciously, John was looking for a woman who is the exact same, she's another kind of fix. As John begins to cry and whispers to Sherlock, "but she wasn't supposed to be like this. Why is she like that?" Sherlock's answer is simply: "because you chose her." Think about all the previous girlfriends John has had over the past two seasons: dull, dull and dull. He can't keep them, not because--as he pretends--Sherlock constantly gets in the way, but because John's desire and need to live that dangerous life with Sherlock gets in the way. He will always choose Sherlock and the game not only because he loves his best friend but because he loves living on the edge.
John follows Sherlock to Magnussesn's--a gun in John's coat pocket just in case--prepared to go along with the deal Sherlock struck if it means he and Mary can live their lives. And then the big reveal--showing Sherlock the Appledore vault. Remember when I said to keep the mind palace in--well--mind? Guess what doesn't REALLY exist? The Appledore Vault. The prized vault full of everyone's darkest secrets is Magnussen's mind palace that he can access anytime he wants but no one else can. This whole venture has been nothing more than playing off people's pressure points. What does Magnussen want? He's a business man. He wants to sell papers and revel in the knowledge that he can control anyone. He has now successfully set up Sherlock and John as traitors to the state. Mycroft, the drugs in his tea having worn off, tracks down his laptop and goes after his brother realizing that Sherlock is about to sell state secrets--at least that is how it looks. It's all leverage. Magnussen wants to own them all. He used Mary to get to John which gets to Sherlock which gets to Mycroft. They are all part of his collection now. Instead of saving John and Mary, Sherlock has fallen into a trap that will take John away from Mary forever.
And this is where we must remember Sherlock's final vow at the wedding. No matter what it takes, he will always be there for John and Mary. No matter what he has to sacrifice, he will ensure that they get their happy ending. With Mycroft descending en masse to arrest John and Sherlock, Sherlock does the only thing he can: destroy the Appledore vaults. While Magnussen taunts the pair about Sherlocks' heroism, Sherlock grabs the gun from John's jacket and says maybe one of the best lines of the entire series: "Do you research! I'm not a hero. I'm a high functioning sociopath! MERRY CHRISTMAS!" and blows Magnussen's head off. And it will cost him everything but the true heroism is that sacrifice. Now that he has committed murder, Sherlock will be taken into custody and John will be free to go back to Mary. Sherlock has to protect them. Recall the very first episode of the series where Detective Sally Donovan tells John, "someday they'll be a body and it will be Sherlock Holmes standing over it. He gets off on this." This is the first time (and probably only time) Sherlock has committed murder but it wasn't in the name of boredom but of love. His unending love for John. Character development. It's a beautiful thing.
Please don't make me wait two years, BBC.
Miscellaneous Notes on His Final Vow
--Well this was a long one. But it was maybe one of the best hours of Sherlock I've seen. This whole season has been nothing but perfection. Normally in Sherlock, there is one episode that resonates with me less than the others. Not this season. Everything was pitch perfect. Awards for everyone--especially Martin Freeman. His portrayal of John Watson has been magnificent to watch.
--The look on Sherlock's face when he makes his decision to save John and kill Magnuseen (the image above) was just heartbreaking. It's a silent goodbye.
"Don't appall me, brother mine, when I'm high."
"Your loss would break my heart" (this one made me just cry and cry)
"Why would I bring a gun to your parents for Christmas dinner?" "So it's in your coat, then?" "Yes."
"Give Mary my love. Tell her she's safe."
--William Sherlock Scott Holmes. My god. Even his full name is sexy as hell.
--Molly broke off her engagement to Tom. (yay!) And YAY for Molly being the one to literally try and slap sense into Sherlock about drugs.
--Random trivia but Sherlock's parents are played by Benedict Cumberbatch's real parents. And Mary is played by Martin Freeman's real life wife.
--According to rumors there will be two more seasons of Sherlock. Praise the lord.
--Is there another Holmes brother? Mycroft made a very passing reference to it. Betcha it comes up in S4 or S5.
--How did Moriarty survive blowing his head off? Your guess is as good as mine.