This was initially published on AO3 and I'm pretty sure I haven't published it on here so I figured why not. Kinda cracky but enjoy. Rated T for language.
Sherlock Holmes wished he could say he wasn't at all surprised the day he opened his private PO box to find not only a letter from Moriarty for him, but for one addressed to Watson as well.
He should've seen it coming, given the rather large painting Moriarty has produced of his partner. Moriarty had, to put it mildly, a fascination with those up to par with her. Up until her arrest it had seem to be limited to Sherlock, however, after Watson solved her, Moriarty's fascination grew.
Sherlock Holmes also wished, to a degree, he was not so intrigued to see Watson's reaction after hand delivering said letter to her.
While Watson had solved Moriarty, he had been unable to solve Watson. He deduced certain traits or factors regarding her the same way he deduced them from other people, but with Watson there was a catch. He cared for her deeply, and that in itself could, at times, hinder his objective deductive abilities.
He had an idea regarding her response to the letter; she would either get offended by the prospect of Moriarty writing to her and would discard the letter immediately, or she would look at it with disgust but keep it and maybe even read its contents.
However, Watson could be unpredictable, whether she admitted it or not. He could very well see her pitching the letter in the fire with a second guess, or refusing to even take it.
It was that unpredictable manner that led him to his intrigued regarding her reaction.
So when he arrived at the brownstone and found her reading on the couch, he forwent warnings and handed her the letter as he explained who it was from.
She looked at it, her facial expression particularly unreadable to him (not too surprising), and tucked it in the back of her book. She took the book with her when she retreated to her room for the night, and Sherlock couldn't help but feel a bit disappointed. While Watson's reaction wasn't one of the choices he initially predicted it might be, it wasn't a surprising reaction. He should've expected that type of neutral reaction to it.
What he did not expect, however, was Watson handing him a letter the next morning addressed to Moriarty while politely asking him to send it with his when he mailed his reply. He stared at it in wonder and couldn't help but want to rip it open to see Watson's reply.
However, he did not, and instead complied with her request and mailed it with his.
A week later two letters were in his PO Box when he opened it, and he gave Watson's hers. The next day she handed him her response.
This routine continued for the next few months. It was a fairly stable routine; there had only been one abnormality to it. The day Watson came downstairs after retreating to her room and promptly placing the letter in the fire. He watched in shock and curiosity, and puzzled over it briefly before coming to the conclusion that Moriarty had crossed some sort of personal line with Joan. He didn't expect Watson to reply, but alas she did.
It was after the few months that one day he decided to ask her about the letter, and, more specifically, her responses to them.
He explained he hadn't asked before out of politeness and respect (and a slight fear of her reaction, in all honesty), but now he couldn't help it. He longed to know.
Watson sighed and leaned against the wall. She began by explaining that the letters were as Moriarty as one might expect; she discussed her art (specifically the portrait of Joan), some of her past, the cases made against her, the deals she was working on, and the unpredictability of 'the mascot'.
"There was one letter though that I couldn't finish." Joan explained, averting Sherlock's eyes. "She…She started discussing having sex with you, and I couldn't read it. That's the one I burned."
He was quiet for a moment before nodding. "What did you say to her."
For a second he swore he could see a hint of a mischievous look in her eyes before she spoke. "The same thing I always say to her."
Sherlock arched a brow, imploring her to continue.
Now Joan couldn't help but let a small smirk glisten her face. "Every time she sends me a letter I always reply the same way."
Joan took a breath. "I say: Dear Moriarty, fuck off. Sincerely, Joan Watson."
Sherlock gaped at her, torn between being impressed and being upset.
"What did Moriarty say?"
Joan shrugged. "She never mentioned it. She keeps sending me letters."
Sherlock struggled to conceal a smile. "Huh. Well," he paused. "Very good Watson."
Joan beamed and gave him that wide grin he wished he saw more. Pure happiness with a bit of smugness interlaced within it.
He let a small smile grow on his own face and the two stood there smiling at one another.
Only they could bond over the cursing out of an evil mastermind.