John almost didn't notice the trigger. Sherlock had been back from the dead for less than a month, and John had moved back in only three days earlier. In many ways Sherlock was the same - aloof, arrogant, sarcastic. But there was a brittleness to him, a sense that if only one knew the right buttons to press, the whole facade would fall apart.

It fell on a Sunday evening. The trigger was, oddly enough, a cigarette.

The November air was chilly and thin, as was typical for the season, but the flat was already stuffy with the accumulated tension that only a Sherlock in a strop could emit. John put up with it for as long as he could, in deference to their still-healing friendship, but eventually he decided the hell with this and opened all the windows he could. Noises from the street filtered in, along with a brisk gust of air and the assorted smells of London in the pre-Christmas season. Someone was standing outside Speedy's with a cigarette and a mobile, and the sharp tang of smoke wafted in alongside everything else.

If John hadn't been watching, he might not have noticed the way Sherlock's incessant flopping about on the sofa abruptly stilled. The change was only conspicuous in the absence of restlessness - where once Sherlock had been in constant motion, muttering and grumbling and flailing about for a better position in which to be peeved at the world, he was suddenly stationary and quiet. John noted this absently, subconsciously, but the importance of the observation grew as Sherlock's unnatural stillness lengthened.

"Sherlock?" John came back to the middle of the room, far enough away to be out of the strike zone if his lanky flatmate decided to lash out but close enough to not be easily ignored. "You okay, mate?"

Sherlock didn't answer, which was a red flag all in itself.

"Sherlock," John pressed. "Something wrong?"

Sherlock's eyes flew open, but he still didn't move a single muscle other than to blink blankly at John.

"Say something," John urged. "You're scaring me."

Sherlock's mouth opened, tongue traced his lower lip, but no sound came out. His chest rose and fall in quick, short breaths. And then, suddenly, John understood.

"It's okay, mate," he said carefully. "Just breathe for me - can you do that? I'm going to - here, press up against my hand." He approached slowly - giving Sherlock plenty of time to signal non-consent - and placed his palm flat over Sherlock's sternum. "Deep breaths for me, just concentrate on breathing. You're having a panic attack."

Sherlock shook his head minutely, annoyance in his eyes, but his breaths slowed and deepened. John slipped down to his knees, the better to maintain contact without hovering or looming. After a minute or two, Sherlock's breathing was nearly back to normal.

"Want me to close the window?" John asked quietly.

Sherlock licked his lips. "No, I-" He closed his eyes for a moment, then opened them again and allowed John to see the vulnerability and embarrassment in the depths of them. "It's fine."

"Doesn't look fine, from here," John countered. "I should know."

Sherlock grimaced, memories of John's many nightmares surfacing between the two of them.

"Should I go?" John hated the thought of being able to do nothing, to not help, but it was Sherlock's call. "I don't want - it's understandable if you'd rather take a minute alone."

"No. You." Sherlock reached up to grasp John's hand, to twine his fingers through John's own. "Missed you while I was gone."

"I missed you too, you git." Understatement of the year. Of the decade. John cleared his throat. "No more cigarettes for you, then?"

Sherlock shook his head no. "Not . . . not after . . ."

"It's okay. You don't have to talk about it."

Some more of the tension bled out of Sherlock's frame. "Thank you."

They were still holding hands, loosely but undeniably. John squeezed, but didn't let go. "Can you - you want to sit up a bit?"

Sherlock's eyebrows drew together slightly, but he shuffled up and forward to make room for John on the sofa. He didn't let go either. John slid in at one end and positioned himself so Sherlock's head could fall back into his lap. It was an oddly intimate position, one they'd definitely never inhabited before, but it was also a supremely comfortable one. Sherlock just let the weight of his head press into John's thighs and settled into the cushions.

They stayed like that until it got dark, until the chill in the air was too sharp to ignore.

They didn't talk.

They didn't need to.