John came in from his weekend with Sarah to see the living room wall covered in blood and Sherlock simply looking at it, his head turned slightly as if examining it. It wasn't that the blood had been spattered—it was put there deliberately, forming abstract shapes that couldn't have been put there by accident.

"Sherlock," John ventured cautiously. "What are you doing?"

"Isn't it beautiful?" His voice was distant and dreamy.

"Sherlock, you're worrying me." He approached his friend cautiously. "Is that blood?"

"Of course…"

John stood between Sherlock and the wall. It was plain that something was wrong. His eyes were clouded and it was hard for him to focus on John's face—his eyes clearly wanted to be elsewhere. They were in the middle of a case, too, and it was a particularly gruesome one. The body had been bled out and then a complex religious mural had been painted on the wall next to the body. The only DNA had been from the victim herself, and Sherlock had been working hard to gather any sort of clue. That was a week ago.

"Human blood?" John wasn't sure he wanted to know the answer, but he'd ask anyway just to be sure.

"Yes." Sherlock still stared at the wall.

"Whose?"

"I don't know, got it from Bart's."

Well, that was a relief, at least. Sherlock hadn't killed anyone. "Are you high?"

"I think you'll find that my bloodstream is devoid of any foreign substance except nicotine." Sherlock put his finger to the wall and drew a soft, gentle line, almost as if describing the outline of a woman. John stared, realizing that Sherlock's bare hand was coated in the blood with which he was drawing.

"When was the last time you slept?"

"I don't know." Sherlock continued to make a mess of the wall by the windows, his strange skull-lamp now on the floor by the fireplace. It was as if he were caressing the walls, rather than drawing on them.

"Have you slept since the case started?" John was irritated at this particular habit of Sherlock's, but had long ago given up trying to convince him that sleep was a useful tool.

"No. I don't think I have." His calmness was frightening.

John gaped. "Sherlock! You need to sleep!"

"I never sleep when I'm working," he said in that peculiar tone. "Actually, I feel marvelous. I can think in ways I couldn't think before, coming up with new ideas all the time, one after another, like a Roman Candle in my head."

The picture was now coming together, such as it was, both the literal drawing that Sherlock was creating on the wall, and the metaphorical one as to what was going on with him. The literal drawing was a recreation of the crime scene, and the metaphorical picture was that Sherlock may have driven himself mad.

"Can't you see it, John?" Sherlock nearly whispered. "Can't you see the beauty of it?"

"No, Sherlock, I can't. You're worrying me."

Sherlock smiled, and under the circumstances it was more terrifying than anything John had seen in battle. "It's a study in scarlet, John."

"What?" John could scarcely believe that Sherlock was comparing murder to art.

"There's the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it." Sherlock did something that cemented John's belief that finally his flatmate had gone utterly mad. He licked the blood from his fingers. Not like a child licks the sticky goo from his hands after eating ice cream, but the lick of someone deriving extreme pleasure from it. Like the way lovers licked one another's hands in the throes of passion. Sherlock's eyes rolled slightly. He was enjoying the taste of the blood.

"Sherlock, if you don't sleep or something right now, I'm going to phone the police!"

"No," Sherlock snapped, and rounded on John, grabbing him by the edges of his coat and shoving him against the wall. "You can't. You mustn't. If I don't solve this, then people will die."

"If you don't stop this, you'll die! The human body can only last so long without rest!"

"But don't you see? I'm so close!" He was shaking John, knocking his head against the wall repeatedly. "Just one more day and I'll know! It's all there, all the clues are right there in front of me."

"Sh—Sherlock!" John stammered, his eyes twinkling slightly from the repeated banging on the back of his skull. "You're not well!"

"What are you talking about, of course I'm well!" Sherlock washed his face off with the nearest liquid, which was, unfortunately, the blood he had been drawing with. He burst into giggles. John dialed for Lestrade.

"Um, we have a problem." Suddenly Sherlock took the phone away and threw it out the window.

"I need to think!" Sherlock stared at the wall, which, John was sure, looked like a complete picture to him, but to his seriously freaked out flatmate, it was a vague sketch at best. "The killer left a message for us, for me, but why can't I decode it? Why wouldn't she just say what it is she wanted? Surely she knows where I live. Shut up, Mrs. Hudson, can't you see I'm busy?"

There was no one there but John. So he was hallucinating now as well. Sherlock added shapes to his drawing, which to John made things less obvious (except that Sherlock was obviously going mad). The doorbell rang and John answered it. It was Lestrade.

"Got your message, or most of it," he said. "What's going on?"

Quietly John responded. "Sherlock hasn't slept since we found the body and he's not doing well. He's hallucinating and drawing on the walls in blood with his bare hands." Lestrade was just as worried by this as John, even though he had yet to see it with his own eyes. The two of them walked upstairs, where Sherlock had expanded on his drawing to include notations on the window panes. Lestrade stood in horror.

"Oh, good Lord," he muttered. "Sherlock, I'm going to have to ask you to come with me." His voice was authoritative, because he knew that this might require force.

"I have to solve this! I have to trace the thread back to its source! I have to!" Sherlock began to pace, his blood-soaked hands dripping all over the carpet and matting his hair where he'd run his hands through it.

"Sherlock, don't make me arrest you," Lestrade warned.

"For solving your crimes?"

"For failing to comply with an officer of the law," he said levelly. "Now if you don't come with me right now, I will arrest you."

Sherlock spun around, his face utterly livid, and only too late did John notice the glint of metal before it entered Lestrade's torso. "NO!"

Lestrade clutched at his wound—it wasn't lethal, but it sure as hell hurt. "A letter opener?" Then he collapsed face-down. John rushed to attend to him, medical instincts kicking in.

"Sherlock, that's enough!"

Cold as ice and twice as dangerous, Sherlock stared at John. "Don't make me hurt you too, John." He turned back to the drawing, Lestrade's blood mingling with the sample Sherlock had been using as the same hand which had stabbed the Detective Inspector now drew furiously.

John turned to see Mrs. Hudson coming up the steps and mouthed frantically at her to call an ambulance and the police. The poor woman was frightened out of her wits, but ran back downstairs to do as she was told.

When they showed up, Lestrade was taken away on a stretcher, John's medical skills having kept anything from going horribly wrong. Sherlock they had to actually sedate, because he threw one attendant out the window and hurled the other three around the room like ragdolls.

"NO! People will die if I don't finish!" He struggled against the firm grip of anyone and everyone in the room, including John. He stared at John with cold homicidal fury. "I thought I could trust you," he spat.

"You can, Sherlock," he said as the physicians forced the needle into Sherlock's vein. "I'm helping you."

"That's what…that's wha…" He finally lost consciousness. John frowned. Hopefully it was only a temporary madness brought on by lack of sleep, but no one was really sure. From now on, John was going to drug his tea every three days.