Disclaimer: Sherlock and Jekyll are the property of the BBC. I own nothing.

A/N: So, the original plan was for this to be a oneshot which I might follow up on if I got the chance. Unfortunately, I forgot to mark it as 'complete' and... here we are! I'm not really sure where this one is going to go. I actually quite like the idea and it's surprisingly easy for me to write. I'll try to do more whenever I have time.

I'm fairly certain this is going to follow the storylines of both Jekyll and Sherlock as closely as is possible. I mean, it's probably going to have a few kinks I'll need to work out along the way (yes, my mind went there too) but I think I can do that. Anyway, enough of that. Read, review, and, most importantly, enjoy!

John started and sat up. He wasn't certain for a moment what had caused his abrupt shift into wakefulness until he heard an ominous creak from downstairs.

He reached for the drawer in his bedside table, aware of where it was without having to look. He opened it and reached inside for his gun. It wasn't there.

His eyes widened involuntarily almost as if they were attempting to take in more light from the darkness around him. His gun - someone had taken his gun. He heard another creak from downstairs and shuddered slightly.

He got up slowly and grabbed an old bent curtain rod from behind his dresser. He gripped it firmly and walked down the stairs, careful to avoid the loose boards that he knew would give him away.

He reached the bottom step and turned the corner, raising the curtain rod above his head. John turned to look the other way, spotted the silhouette of a tall, thin-limbed figure, and automatically started to swing the rod before realizing his mistake and quickly stopping his movement.

"Jesus, Sherlock," he said, in an embarrassingly breathy voice. "The next time you're going to sneak into the flat in the middle of the night, would you like to let me know?"

The figure turned to look around at him and John blinked in confusion, fighting the urge to raise the curtain rod again.

"I'm not Sherlock," a lilting Irish voice said. John shook his head.

"No," he said in agreement, "you are most definitely not Sherlock. I'm sorry, I - well, you look exactly like him. And it's, um, it's his flat so I sort of assumed. So... who are you?"

A long, spindly arm reached out and flipped the light switch. John looked and saw a man with dark curls and matte black eyes, dressed in a sweatshirt and jeans - both clearly new.

"My name is Carl, Carl Holmes. I'm Sherlock's cousin."

John frowned. "Sherlock's never mentioned you, did he know you were coming round?" he said dubiously, looking Carl up and down.

He did look almost identical to Sherlock - almost too similar for a cousin but there was no other explanation. If there was another brother he was certain that Mycroft or Sherlock would have told him.

Carl said, smoothly, "Yes, I let him know. Do you have any idea when he'll be back?"

John shook his head and then, with a shrug asked, "How long will you be staying?" Carl gave a small smile that looked a bit too much like a smirk for John to be entirely comfortable with it.

"Oh," he said, airily, "I'll probably be in and out for around a week. Unless, of course, you object?" He raised his eyebrows, as if to show exactly how unlikely he thought this was.

"No," John said, feeling slightly unsettled in a way that was... not entirely unpleasant, "I don't mind." He looked down and saw that he was still holding the curtain rod. He blushed and set it down awkwardly.

"Uh, would you mind taking Sherlock's room for tonight?" he asked. "He never sleeps in there, so I'm sure everything will be clean. He won't mind, he usually sleeps on the sofa anyway."

Carl gave a slightly crooked smile and a nod. John felt a slight flutter in his stomach. It was a very attractive smile. He quashed that thought before it could turn into an action he'd regret in the morning, and turned to lead his strange new house guest back to his room.

Sherlock awoke the next morning in his own bed. This would not have been an oddity for anyone else, but for Sherlock, it was almost unbearably disorienting. He started and thrashed his way out of the bed in a way that was certainly not dignified by any description.

The impact when he hit the wooden floor jolted him completely awake, and with an sharp gasp, the memories of last night came flooding back into his mind. He shook his head, as if trying to get water out of his ears.

"Oh god," he mumbled, remembering the strange man - Moriarty - who was him and not him and who'd threatened... He'd threatened to come after John.

He pulled himself off the floor and ran full-tilt into the hallway. "John!" he yelled in a panic, "John, where are you?"

"Sherlock?" John's voice came from the kitchen, sounding confused but unharmed. Sherlock breathed a sigh of relief and leaned against the wall. John came around the corner, coffee mug in his hand and an expression of bemusement plastered across his face.

"I didn't hear you come in," he said. "Has Carl left?"

Sherlock blinked. "C - Carl?" he said, shakily. John frowned at his flatmate. "Yes, Carl. Your cousin? He said he told you he was coming. Has he gone?"

Sherlock's mind raced but he nodded, nonetheless. "He went out early this morning. He told me not to wake you."

"Hmmm," John said, "I've just brewed some coffee, do you want some?" He turned to go back into the kitchen.

"Yes... black -"

"Two sugars. I know."

"Ah. Thanks. Could you just leave it on the counter for me? I need to pop back into my room for a minute."

Sherlock heard a vague sound of assent from the kitchen and headed back into his bedroom. He shut the door and sat heavily on his mattress.

"Carl," he muttered, "that bastard." He pressed the heels of his hands into his eyes and tried to ignore the stinging he felt there.

Time to assess what he knew. His alter ego - Moriarty, or Carl, as he was now calling himself, had paid John a visit last night but hadn't harmed him. He might be planning to come here again in the future. But what could he possibly gain from it, apart from antagonizing Sherlock? What was in this for the psychopathic mastermind?

He wracked his brain for any traces he might have left behind...

His mind was like a network of tunnels, all branching off one central room. That was the room where all his core data was stored - how to breathe, how to sleep, how to think. It was a bright white room, lit with fluorescent lights. There were no secrets there.

The tunnels were shadowy and vague, edges blurred, walls not as defined as they should be. They were filled with doors. Most of them he could open with ease because they were familiar to him, like the room where he kept his methods in. Others jammed and he was sometimes unable to access them, like the room where his emotions lived. That room was filled with whispers and a faint pulsing, like that of the heart he didn't really believe that he had.

What was he looking for now? He was looking for a locked door. A door that he didn't want himself to find.

Back in his bedroom, Sherlock smiled and steepled his fingers.

He was definitely more than equal to this task.