For the point of sanity, the John that just burst in will be referred as Other John, and dying John will be referred as Normal John.
Everyone stared at the newly appeared John. Doorway John got right down to business, seeing as everyone else was busy questioning his existence.
"Right, well. This is awkward. I'm John, I mean, I'm Other John. Hi John!" Other John waved attentively at Normal John. Normal John stared at him.
"Normal John, because that is what I'm calling you now, you have to talk now."
Normal John stared at Other John, before offering a somewhat nervous "Hi Other John."
Other John seemed to remember something, before suddenly bursting words out at Sherlock.
"Sherlock! Decrease the molecular structure of Eldox Methrylite by the power of three, and combine with stardust!"
Sherlock looked at him with a look between confusion and bafflement. "What?"
"The cure." Was all Other John said.
"Of course." Sherlock's eyes suddenly burst with intelligence and activity. His eyes moved rapidly from side to side, like he was speed-reading invisible text, and he stood perfectly still. He rested his hands on his head as if it would speed up his thought process, and seemed to stare at nothing. The Doctor opened his mouth to talk, but Sherlock instantly shushed him, busy in his mind palace and demanding silence.
"Doctor! Get me Eldox Methrylite, a Sonic Screwdriver, and a Picascope! Now!" Sherlock shouted, but not taking his eyes away from that distant place in his mind.
"What's Eld-" the Doctor started, a frown on his face, before Sherlock interrupted him.
"The purple crystals you got from BallaValley."
The Doctor ran out, and Sherlock thought.
"Wow that is creepy." The new John said.
"What?" the Doctor said, obviously confused.
"You just said that. And now you're saying it again."
"Saying what?" the Doctor asked.
"That."
"What?"
"And that."
"John, you're not making any sense." The Doctor said.
"I'm making perfect sense, you just aren't keeping up."
The Doctor gaped; outraged that John had just used his own words against him. It stung.
"John, you have to talk now." The John in the doorway said.
"Um… hi John." The John on the ground said.
"Hi John." Other John said, with a little wave.
"I don't mean to be rude, but why are you here?"
"Oh, lots of reasons." The new John said.
"But mainly, I'm here to replace you."
Sherlock was suddenly pointing the gun at the John in the doorway.
"Sherlock, put the gun down." The Doctor said slowly.
"No."
"Sherlock!"
"He's not replacing John."
"Sherlock, he is John."
"How do you know that? What if he's some sort of Teselect, or doppelganger, or a Zygon?"
"He isn't."
"You don't know that."
At that point, Other John interrupted their conversation.
"I'm really not a Zygon or a dopplethingy, or whatever you said I am. But if you want to kill me, you'll be killing Normal John."
"How so?"
"It's… wibbely-wobbil-something…" Other John said, creasing his face in concentration.
"Wibbely-wobbily-timey-wimey!" The Doctor said, excitement in his voice.
"Sherlock! I can fix this. Just cure John, and I promise I will fix this!" the Doctor said, beaming.
Sherlock relaxed. "On your head be it." He said, and he turned back to the kitchen to prepare his formula. He took the modified Eldox Methrylite, and he crumbled the stardust in, watching it dissolve with keen eyes. When he was satisfied, he took a syringe from his pocket and he out it in the test tube, filling it with the indigo liquid. He then took it over to Normal John, who was lying on the floor with his eyes closed.
"John?" he asked quietly.
"mh?" John mumbled weakly.
"John, I'm going to inject this formula into you now, okay?" Sherlock said softly, being careful to stay clear from Normal John's arms, which were starting to feel around in search of prey.
John gave the smallest of nods, and Sherlock bent down and injected the liquid into the side of John's neck. John gave a slight gasp, and his eyelids fluttered slightly.
"John?" Sherlock asked with concern.
John did not reply, and the Doctor reached forward, putting his hands on Sherlock's shoulders.
"Sherlock, you need to get up."
"But John might-"
"He'll be fine. You've done all you can, now we just need to wait. Come on."
"Okay."
Okay, so I know it's quite bad. But I just needed to post something. I'll probably re-write it soon. But yeah. There you go. I have no idea what I am doing. Help, possibly...