Over at Tumblr, I've been speculating about a new theory I have about Mary Morstan's past. I thought some of you not on that site might enjoy it. Series three spoilers throughout.
In my comments on a recent photo I shared, I mentioned a theory I had that Mary was involved in some way or other in engineering the showdown on St. Bart's at the end of TRF. There are two possibilities in that scenario: either she was working with Moriarty somehow, manipulating someone in a way to drive Sherlock to that final encounter... or (assuming there's any truth at all to the final story Sherlock tells in TEH) she was working with Sherlock and Mycroft to engineer the whole thing. This is the unforgivable sin that Mary thinks will make John not love her: that she knew Sherlock wasn't dead and didn't tell him.
There's no guarantee Mary had anything to do with the Fall, but it makes sense she would be. That's the one thing, more than any other, I think would make John absolutely hate her. Working with that assumption, I think it makes a lot of sense that she's actually working with Mycroft and Sherlock. Why? Because the one thing we know about her with any degree of certainty at this point is she's worked for the CIA.
This isn't the first time the CIA has made an appearance in the show. Most obviously, there's the American in ASIB, who tries to "steal" the photos from Irene Adler's home and later attacks Mrs. Hudson. Then there's the HOUND project, which Wikipedia described as a CIA operation. (I'm not sure what the source is; I haven't rewatched those scenes to verify because I'm tight on time.) In both cases, the people with the most obvious connections to the CIA (the American, Dr. Frankland) are also directly connected to Mycroft. Now we have a third person with a CIA background. We all know what the Holmes boys say about coincidences.
There's another data-point in this theory's favor, if you read HLV carefully. According to Ariana DeVere's fantabulous transcripts, in the scene after the Leinster Garden showdown, where Mary gives John the AGRA jump-drive, she says that the stuff Magnussen has on her could send her to jail for the rest of her life. But that's not actually the threat Magnussen makes at the end of HLV:
It works like this, John. I know who Mary hurt and killed. I know where to find people who hate her. I know where they live. I know their phone numbers. All in my Mind Palace - all of it. I could phone them right now and tear your whole life down - and I will ... unless you let me flick your face. This is what I do to people. This is what I do to whole countries... just because I know.
Having a wife outed as an international assassin and sent to jail is the kind of thing John thinks would tear his life inside out, but that's nothing compared to having Mary's victims come after both of them. For a professional killer, Mary's actually pretty discerning in her targets, or at least she makes a principled case for why she went after Magnussen. A self-serving case, but principled nonetheless. This makes me think that the people Magnussen is imagining telling this to aren't grieving housewives and schoolteachers. We're talking about al-Qaeda, the Mafia, that kind of thing. Crime bosses and enemies of the state. Even if John could survive physically there's no way he could stay in London, or even England.
But remember why Magnussen wants to put pressure on John in the first place: as a way of controlling one Mycroft Holmes. Blowing Mary away takes that pressure point away. Forcing John and Mary so far underground that Sherlock is helpless to assist them significantly weakens it. Having Mary sit quietly in jail, and John sit quietly in the London suburbs or 221B or wherever, gives him significantly more power. Perhaps he makes the most ruthless threat he can because he's trying to scare John into submission. But another explanation: Magnussen just assumes that he can't send Mary to jail just by dropping information on her, because she has some kind of immunity, diplomatic or otherwise, due to her work with Mycroft. He wouldn't tell John this because as far as Magnussen knows, John is in the dark about the full extent of Mary's past - and he doesn't want to hurt John, but rather maintain the status quo. Pushing John away from Mary to Sherlock would be counterproductive.
Reason #3 why I find this idea compelling, aside from the fact that I think it would make for a pretty interesting series four: it actually makes some sense of how little Sherlock suspected her earlier on. He sees she's lying. Fair enough; I'm sure most of John's dates, or the people he meets generally are lying about something or other. But then we see him being so ready to trust the fact that Mary will help him. He doesn't express particular surprise when she works out that John's in danger because of the cypher text she receives. He seems ready enough to think Lady Smallwood is the assassin in HLV, but when it's Mary, it blows him away. To Sherlock, it seems like a deep, deep betrayal, and one he just didn't see coming.
Again, the surface explanation, the obvious explanation is that Mary is very, very good at deceiving people, and that Sherlock's off his game because he's been away and is traumatized by his undercover work. At some level, that makes a lot of sense and gives us an interesting character arc for Sherlock throughout this series. But the more I think about it, it seems even more likely that Sherlock's not evaluating her the way he would a stranger at all. He thinks he knows her, and he thinks he can trust her to be deceiving John but not him.
In my opinion, this idea has the ability to put quite a few scenes in a different light. Some things it helps with, like explaining why Sherlock wasn't more on his guard around her and why he was able to be fooled so thoroughly by her. Other issues it exacerbates, like why Sherlock would let his best friend marry her without giving him a heads-up. I'm not pretending it doesn't have troubling, even implausible implications for how events unroll in series three. I may come back to those if I have time in later posts, though I'm hesitant to speculate too much. Coming up with theories in the absence of evidence is the quickest way to mislead ourselves, if certain consulting detectives' Victorian doppelgangers can be trusted.
That said, I think for me the biggest impulse driving this theory is that Mary just seems too convenient of a villain. She's Snape at the end of Half-Blood Prince, so dastardly standing there over Sherlock while he bleeds out that she couldn't possibly be anything other than Moriarty in heels. Series three is a story arc where nothing is as it seems or as we've come to expect: not John, not Sherlock, not Anderson, not Mycroft, not the Holmes Sr., not Mrs. Hudson, not Magnussen, not Moriarty. Are we really so sure that Mary will be an exception?