The letters start just after Joan moves back into the Brownstone.
My dearest Watson...
She crumples the first one up in her hands, shaking with what might be anger. How dare Moriarty assume to know her? How dare she intrude? She gets halfway up the stairs to share her rage with Sherlock when she stops, smooths out the letter, reads it again. She bites her lower lip and glances up the staircase. Why bother him? His relationship with Moriarty is complicated – twisted – enough already, and any threat to Joan brings both his protective instinct and his coldest rage.
She folds the letter, neatly this time, and slips it into her pocket. It's not like she has to tell him everything, they're not a couple. (That they are rather more than a couple is a thought she pushes to one side.) She climbs the remaining stairs taking calming breaths.
Leave it. It's nothing. Don't give her the reaction she so obviously wants.
The letters continue despite Joan's lack of response. Chatty gossip about prison, polite inquiries as to her health, the occasional joke. She finds herself re-reading them by the light of her bedside lamp, when the house is as silent as it gets. She designates a shoebox to the letters, doesn't bother with anything more secure because Sherlock would never so much as think of going through her possessions.
(Yes, Moriarty writes, he can be tediously predictable at times.)
What Joan should do is burn the letters and find some friends who don't dabble in detective work.
What Joan does do is reach for paper and a pen.
They're becoming quite close, in a way. It's good to have someone she can vent to, someone who is safely locked up in prison and universally distrusted. Even if Moriarty produced the letters as proof, everyone would assume that they were fake. She was, after all, a forger for a time.
Moriarty – Jamie, as she's asked Joan to call her – introduces the eroticism slowly. A joke here, a sly innuendo there. The letters become filthy by degrees so subtle they are impossible to retrace. It takes a while for Joan to reply in kind but eventually, of course, she does.
Where's the harm?
She doesn't trust Jamie, and she certainly doesn't love her, but she plays along. When her pen-pal assumes too much she scribbles an angry I am never writing to you again before scoring that out and replacing it with I'm not attracted to you.
Is that why you've never told Sherlock about my letters? Semi-colon, right bracket. Jamie Moriarty has written a smiley face. This is somehow the most disturbing thing her correspondence has contained.
Joan resolves not to reply, and manages to keep her thoughts from paper for a full six months.
She awaits an apology she doesn't really expect to receive. Instead:
Tell me how you'd plan the perfect murder.
Joan Watson is indignant. I would never murder anyone, you should know that.
Of course I know that, my dear. But if you were going to murder someone, how would you do it?
As it transpires, this is something Joan has thought about before. Daydreams, idle moments soaping her arms in the bath, occasional dark thoughts about someone who was rude to her. She shares the information because she wants Jamie's input. She'd like to know how clever she really is. Who doesn't?
Jamie is delighted. If you ever want to work for me, just say the word...
Joan recognises the signs of her own murder plot when she sees them. She steps back from the mortuary slab and covers her mouth with her hand. She runs out of the room, choking.
In the corridor outside, Sherlock hands her a plastic cup of water. “That wasn't even the most disgusting corpse you've ever seen,” he says, with a hint of concern. “Perhaps you're coming down with something.” He lifts his hand towards her like he's going to take her temperature, but then he stops, frozen for a moment, and retreats from her glare. His hands fidget at his sides.
“Do we have any suspects yet?” she asks, as though she didn't know.
“As I'm sure you're aware, Watson, there is no such thing as the perfect murder. A clue with reveal itself in due course.”
Joan nods, distracted. It's not her fault, she didn't kill anyone. Nobody could blame her for this. She is not a murderer.
These things are true, but she doesn't believe them.
The crucial clue does not, in fact, appear. After three days without sleep Sherlock declares it unsolvable and sulks for a week. Joan tidies the case files away in a folder and breathes a sigh of relief. Not that he would blame her, but she blames herself and that's bad enough.
She starts writing to Jamie again, does not mention the murder even in cloaked words. They pretend that nothing has happened but they both know Joan has fallen in her own estimation. Maybe that was the plan. Maybe this was always going to happen when Jamie wrote that first letter.
She wonders what's next.