When Joan gets home from the dry cleaner's - a real dry cleaner on East 34th she found while working a case - Sherlock has a board with pictures up in the living room. He's completely ignoring them for a shot on his smartphone, and Joan leans in to squint.
A two of clubs and a seven of hearts, with green felt behind. The whole thing's adorned with what looked like arterial spray. Maybe, if the victim was sitting in the right spot--
"What do you see, Watson?"
"I'm working on it," she murmurs.
"Not about the blood pattern." Of course he knows where her eye would go. "The cards."
Joan snorts. "I think whoever had that hand should have folded right away."
Sherlock drops the hand with the phone into his lap. "And how do you know that?"
"It's Hold 'Em. Everyone knows that's the worst hand." She shifts the bags of clothes draped over her shoulder. "I'll be upstairs."
Joan makes it all the way up the stairs and into her room without interruption. She pauses in the door. Sherlock had had that gleam in his eye, that little smug look that said he was going to run off to Gregson and give him a suspect or the story or both. But he didn't stop her from leaving.
Before she can doubt what she saw, the front door slams.
Because Joan takes a cab, she's behind the mirror when Sherlock and Gregson confront a white man, probably in his 40s, in a polo shirt that makes him look like a tech nerd.
"...knew that it was the worst possible hand in Texas Hold 'Em. But he was betting like it wasn't, wasn't he? And you should have had no way of knowing that he was bluffing."
Gregson slips a file across the table. "Except you did. We found this camera set in the frame."
"So everyone folded, he didn't show his cards, and when everyone left...that was it. It looked like a crime of passion, of anger, but no one witnessed it because you waited until they were gone to turn off your camera. And all the cameras in the room in one motion, actually. So it looked like you left on the footage they had."
The man opposite Sherlock shifted in his chair. "I won't speak without my lawyer present."
"Suits me," Gregson said, flipping the file closed. "I'm sure the judge will listen to every word he says."
Joan slips out of the observation room just before Sherlock can leave interrogation, but he doesn't look surprised to see her. He also doesn't let her get a word in: he says, "Watson! Just the person I wanted to see," and turns for the elevator.
She doesn't give up that easily, of course.
"You cut me out of this one completely," she said as they walked toward the elevator at a brisk pace. "Why?"
"You have worked cases without me. It's sometimes prudent to work apart."
"Not right now."
"Yes, well." Sherlock pushed the elevator button. "Our combined efforts did solve this case rather quickly, even if you came in at the end, and I have something else that needs your rather unique attention now that our time is free. Shall we?"
Joan crossed her arms, but as the doors opened, she stepped inside.
"Is a poker table, yes."
"...wasn't here before," Joan said. "I know it's a poker table."
"Of course you do." Sherlock sat on the opposite side from the dealer's seat and gestured toward the empty chair. "Would you do the honors?"
Joan lowered into the chair. "What do you want to play?"
"I think we established that this is not a blackjack table, correct?"
Joan rolled her eyes and picked up one of the decks. Seal wasn't broken: it was fresh. A quick look at the other decks stacked told her they were the same. There was even a discard holder similar to those in major casinos, probably for the sake of neatness more than anything.
She dealt the cards while Sherlock pulled equally fresh chips out of their boxes.
Sherlock was obviously new to the game. Obvious for Sherlock, anyway; Joan had been around him long enough to see him pick up a couple new subjects for various case work, so she knew what it looked like. He'd breeze over the frequent losses at the beginning in the interests of getting the hands faster, and as he got better and won more hands, the losses frustrated him more.
Except, this time, the gleam was in his eye every single time he lost.
They played through the night, and Joan's stack of chips only dipped in a visible way when Sherlock lost all his holdings and demanded Joan split the chips so they could start again. It wasn't so much that Joan was dominating the hands; she lost a statistically average amount, probably. It was that she knew which hands were more likely to win, so she folded when she had to and lost very small amounts. Sherlock, on the other hand, never folded, and he always until Joan called.
Finally, when Joan could hear the morning traffic outside, Sherlock said, "Atlantic City for your twenty-first birthday."
"Vegas," Joan said. "And it wasn't just twenty-one. I took a pretty big trip when I got accepted into med school."
"Of course. And once you were working at hospital, it was probably prudent to have poker nights with your colleagues, since you all worked the same unreasonable hours and couldn't get away for social opportunities."
Joan smiled. It wasn't that hard to figure out, but she did miss those nights.
"Just because we played all night doesn't mean you know my tells," she said, standing and stretching with a groan.
"No. But I have a start."
At the same point next week, the table was set up downstairs again. This time, Ms. Hudson, Alfredo, and Carrie filled chairs sat around Sherlock. Carrie and Ms. Hudson were having an animated discussion about something, and Alfredo was flipping through his phone.
Joan raised an eyebrow at Sherlock.
"I might know a couple of your tells," he said, face carefully neutral.
She gave him one more look, and then she turned to the rest of the table, beaming.
"Who's dealing?" she asked.