Culverton is repulsive, evil, and up to some creepy medical experimental shit, based on setlock + canon. His show was also on a hiatus, as setlock posters show us he’s “back.” We know we’re getting some AGRA history in s4, so there’s a decent chance their pasts are intertwined.
Speculation and crack for fun and profit…
Five years ago
AGRA was on assignment to kill Culverton. Maybe for Mycroft. Maybe for no one but herself. “People like Magnussen should be killed. That’s why there are people like me.” And Culverton, apparently, qualifies as “people like Magnussen.” Even more repulsive, according to Mofftiss.
Clearly, she fails. Culverton’s still alive, right? But AGRA is a badass at what she does. Hard to believe she just…can’t kill him. It’s more complex than that. Let’s say he knows she’s coming for him. Let’s say he sets a trap, like the one canon Culverton set for Holmes in The Adventure of the Dying Detective. A spring with an infected tip loaded in an otherwise empty box. The “treasure” of/for AGRA.
It works. She’s infected. Now she has to keep Culverton alive, because he claims to be working on a cure.
In the meantime, onto AGRA’s next assignment: becoming Mary Morstan, soon to be Watson. Maybe with the help of Mycroft.
And Culverton goes off the grid. Maybe with the help of Moriarty.
And Magnussen sees all of this and stores away the information for future blackmail.
Magnussen decides it’s time to let this incredibly dangerous woman know about the power he has over her. He sends “Mary” oodles of love and big squishy cuddles. He calls her…
Poppet. A term of endearment. Also a doll used in witchcraft to transfer harmful spells to the person they represent. A voodoo doll. An imposter.
AGRA doesn’t appreciate this reminder of her Culverton situation on this particular day. She doesn’t need a reminder that her illness is progressing––she sees the signs.
So does Sherlock.
“Increased appetite, change of taste perception, and you were sick this morning. You assumed it was just wedding nerves. You got angry with me when I mentioned it to you. All the signs are there.”
He misdiagnoses the signs as pregnancy. John wonders how he, a doctor, could have missed seeing that his own fiancée is pregnant. (He didn’t, because she isn’t.)
….concerned, to say the least. Her days are numbered unless Culverton comes up with a cure, and she knows it. And now, thanks to Sherlock, she has to fake a pregnancy until she can figure out a better long-term plan.
But she, as Sherlock deduced immediately, is shortsighted. Bringing us to….
AGRA risks everything, confronting Magnussen for her files. Magnussen knows everything about her––including the fact that her last mark was Culverton, that she didn’t kill him, that he infected her.
And she’s not just trying to get her files from Magnussen. She’s trying to find out where Culverton is, whether he’s found a cure, if he’s even working on it. Her time’s running out. She can’t actually fake a pregnancy once she’s supposed to start showing. She’s married. To a doctor.
Once again, Sherlock fucks everything up. Offers to help her. Like he helped with the “pregnancy” deduction? Ugh, fuck this. AGRA shoots him and bails.
He survives, John finds out, he and Mary are separated till Christmas. Well, that makes faking the pregnancy easier for AGRA. While Sherlock recovers and focuses on Magnussen, and John (supposedly) hems and haws over whether to read the flash drive she gave him, AGRA sets her sights on Culverton.
(If Moriarty’s alive, this is when he and AGRA are reunited. If Moriarty’s dead, she’s acting alone in his place.)
She finds Culverton. She threatens him. Time to come out of hiding and finish what he started.
Christmas and the tarmac
Back to being “pregnant as an elephant” Mary Watson. Thank god for cold weather as an excuse to wear layers, and a husband still so “basically pissed off” that he’s not up for much touching. Here’s a scan, looks like a girl. Yay, domestic bliss restored. AGRA is about as thrilled as John.
Days later, they watch Sherlock’s plane take off. But then Moriarty’s face is everywhere, and Sherlock is about to fuck everything up for AGRA yet AGAIN. Because as he rapidly tries to figure out what the fuck is going on with Moriarty, he’s also figuring shit out about “Mary.”
“She was dying.”
Sherlock is momentarily thrown off, because the bride in his simulation is Jim Moriarty, but Jim Moriarty wasn’t dying. That’s not why he shot himself. What the hell is Watson on about here?
Oh, that bride. The actual bride.
John’s always right. He didn’t see signs of pregnancy, did he. And wasn’t that odd? Considering…well, how much more intimately he knew Mary than Sherlock did? Now Sherlock understands what his simulation has revealed. Mary…AGRA…is dying.
“So she decided to make her death count.”
Culverton’s back. Sherlock investigates––maybe even at the behest of “Mary.” He’s onto her now, and puts the pieces together as he tries to stay a step ahead of her. After everything that’s happened, maybe he really is still trying to help her, to see if there’s a cure. But AGRA knows exactly how repulsive Culverton is. She knows her days are numbered. And she’s got no problem putting Sherlock in this man’s path.
I’m ending this here, because there’s a gazillion ways this could go, and I haven’t even touched on how Culverton might tie in with Moriarty and Redbeard and the Other One. But right now, I’m more convinced than ever that there’s no baby in s4 outside of John’s nightmares in e1. And I know this theory is super cracky but I vastly prefer it to the alternative (ie, the baby dying).