I'm surprised that no one has done this one yet.
A common sight in people with PTSD is an aversion and fear of fireworks. They associate the sound with explosions. Its something that I think is really sad.
It was a cold November night and the residents of 221B were curled up in their flat, avoiding the cold of the outside.
Sherlock was dressed in his pajamas in bathrobe, sitting in his armchair and reading a rather large and dusty book. John was sitting on the couch with his laptop, mostly wasting time answering emails.
There was someone firing off fireworks nearby, the blasts were sounding off loudly and color reflected through the curtains into their living room. Sherlock was mostly blocking out the sound, focusing in on what he was doing. He had the ability to block out any outside noise if he wanted to.
Any noise except the one that was now making itself known.
John had whimpered.
Sherlock's eyes flicked up from his book, and he looked over to where John sat. The doctor was still staring at his computer screen, but every time a firework erupted into the sky he winced. The wince would stay on his face giving him the look of someone in constant pain. Then as the next firework went off, a tremor shook John's entire body and he gave a small sound of terror. Closing his laptop, attempting to look casual about it, he made a move to leave the room.
Sherlock stood and intercepted him, wrapping his arms around the man and pulling him back onto the couch. He pulled John into his arms and pressed the soldier's head against his chest.
"It's alright." He murmured. John was shaking, and clenching his teeth.
"What...what do you mean?" He asked, still trying to play it off.
"We're still in London. Still in the flat." Sherlock spoke in a soft calming tone, rubbing small circles onto John's back with his fingers. "You're not on the battlefield. You're here with me."
John froze as another explosion went off in the sky. His hands grabbed the back of Sherlock's bathrobe and gripped the fabric tightly.
Sherlock pressed his hands over John's ears, while his eyes searched the room for a more permanent way of blocking out the sound. His eyes fell at last on his violin.
"Stay here." He ordered, slipping out from underneath John who was nodding and squeezing his eyes shut. Sherlock picked up the violin and bow before returning to the couch. He then attempted a posture that most violin players would not, keeping John pressed against his chest while also leveling the violin to his shoulder. He began playing as loudly as the wooden instrument would allow, he didn't attempt to play anything happy knowing that soldiers were not consoled by constant shows of the good in the world. Instead he played a more somber tune, he wanted to give John something to relate to.
The army doctor was gripping the front of Sherlock's shirt, pressing his face into it as if trying to block out the world.
"Get down." He warned suddenly and frantically. "Sherlock, get down. Don't get hurt."
"It's alright." Sherlock reminded. "We're in London. We're not in danger."
He kept on playing long after the fireworks had stopped, and long after John had drifted into sleep. Finally he lowered the instrument carefully to the floor so he could hold John in his arms. He pressed a kiss to the doctor's slumbering head.
For the rest of the night the detective kept careful watch over the man who was tormented by fireworks and dreams of explosions.