The door to 221B burst open as John walked up to it, almost making him jump in surprise.

"Good god, Sherlock, what's got you—"

"The umbrella, John," Sherlock said loudly and firmly. "Put it down right now."

John dropped it without question. "What is going on?"

"What are you feeling?" Sherlock demanded, taking John by the shoulders. "Physically, what are you feeling?"

John shook his head in confusion, but paused to take a mental inventory. "I suppose…my arm feels a bit tingly, I don't know, I didn't think it was—"

"Do you remember me telling you the symptoms of pseudaconitine poisoning? Is that what you're feeling now?" Sherlock still had a firm hold of John's shoulders.

"Yeah, yeah, I do. Um—" He stopped. His confusion about Sherlock's behaviour suddenly evaporated. He looked Sherlock in the eyes and felt a stab of fear in his gut. "It was on my umbrella, wasn't it?" he said, his voice surprisingly calm.

Sherlock gave the faintest nod.

With that, John's knees buckled and he collapsed.

Sherlock heaved John into the flat and left him on the floor in front of the doorway. John was still conscious, but he was going into paralysis.

"Oh, John!" said a fearful Mrs Hudson from behind Sherlock.

"I'll call an ambulance," Mycroft said, infuriatingly calm.

"No," Sherlock snapped at his brother. He was looking frantically around the flat as he said, "You could but he doesn't have time."

Sherlock stopped for a moment and mentally panned through the hundreds of things he knew about pseudaconitine.

"Atropine!" he burst out. "I need atropine!"

Sherlock knew immediately that there was no such drug in the flat.

He was wrong.

"My room," John managed from the floor. "Bedside cabinet."

Sherlock didn't question. He had gone up, got the ampoule and a syringe, and come back down to John's side in less than thirty seconds.

The only sound Sherlock could hear while he was swiftly drawing the atropine into the syringe was his brother talking on his phone down the stairs. Otherwise, the room was silent.

Sherlock felt John's pulse to ensure that it was safe to give him the drug and slowly injected it.

The next minute passed without a word from anyone. Mycroft re-entered the room and Mrs Hudson must have still been there, but Sherlock took no notice of either of them.

John twitched slightly. He took a sharp breath and sat up suddenly. He looked dizzy and slightly confused. He glanced around and found Sherlock's face.

"I…." He looked around again. He seemed to reorient himself and understand what was going on. "One of the side effects of atropine is nausea," he said vaguely, and threw up in front of Sherlock.

A rush of relief flooded through Sherlock. He hadn't realized he'd been so rigid, but he finally relaxed. He could hear sirens outside as the emergency vehicles stopped on the street in front of 221B.

"So you figured it out, then," John said, lowering himself back to the floor. "You can stop worrying about the case."

Sherlock snorted quietly. "Right now, John, that's the last of my concerns."