Watson stood, both hands resting on his cane. Or rather, both hands clenched white-knuckled to the cane-top, all his weight leaning forward on it as though to prevent himself from raising it. He was trembling, faintly.
Sherlock was rather impressed, really. An excellent demonstration of control.
"Holmes," John said, through a locked jaw. "Could you say that again?"
Sherlock opened his mouth, but the sudden glare directed his way shut it again. He blinked. Well. If you don't want something, one would think you shouldn't ask for it.
"You were testing the buoyancy of a body, you said," John went on. So stiffly, rigidly calm that Sherlock was, in fact, beginning to worry about his teeth and jaw-bones. "A child's body. By throwing him in the river. Yes?"
"Yes," Sherlock shot in, quickly. "It was perfectly fine, you know. He was only a little drowned, just enough for me to get ..."
"Sherlock," John cut in, and it may as well have been a saber. Sherlock noted, absently, that the knuckles about the cane-top had grown impossibly whiter. "Robert cannot swim."
"I know." He was getting the impression that this was one of those rare moments when he really, really should not keep talking, but if John only understood ... "I needed to judge times. Swimming would have complicated the matter." Silence, thick and deadly. "He agreed, you know. Robert. He wanted to help. He ..."
"Holmes." And Sherlock felt a chill, the likes of which he hadn't since last he'd faced Moriarty himself. "I want ... I want to make one thing perfectly clear, alright?"
For the deadly, manic light in John's eyes, the rictus grin that was edging onto his bone-pale features, Sherlock nodded hurriedly. He had a sudden idea of what Moran must have seen bearing down on him. Or any number of Afghani soldiers, too.
"You know I'm aware of your methods, don't you?" John asked, and it was oddly light, encouraging. Sherlock blinked, and nodded. "And you know that I am a doctor, yes? As well as a soldier?"
"Ye-es?" Sherlock asked, somewhat hesitantly, as John stepped up to him, and rested one light, congenial hand on his shoulder. "One of the best, Watson."
John smiled, and Sherlock had seen friendlier smiles on men about to stab him. "Thank you," he said, still alarmingly genial. "Then you'll understand this."
His hand clenched, suddenly, around Sherlock's shoulder, bit into it with all the force of a vice crushing closed. Sherlock, very carefully, fell still, staring in wide-eyed appraisal at John.
"If you ever involve my son in one of your experiments again," John told him, low and emphatic, "If you ever put him in even a second's danger. I shall find so plausible a means of killing you that you would not realise you had been murdered." He smiled, still light, still amiable. "I will offer up a cause of death, and no-one, not even Mycroft, will think to question it, it will be that subtle. Are we clear?"
Sherlock, trying to contain the sudden surge of something that felt alarmingly like lust, nodded. John, smiling lightly, let go of his shoulder, and raised his cane to tip his hat.
"Good," he said, as if he hadn't just threatened murder in truly spectacular fashion. "You can content yourself with experimenting on my dog, from now on, then." He tipped his head at Sherlock, mild and unassuming, and smiled. "I'm glad we had this conversation, Sherlock. Now I can tell Mary to relax, hmm?"
And Sherlock, watching the line of his back as he departed, could not help but feel a small thrill of pride. Well, smugness, really.
Good Lord, but he had chosen well, in Watson, hadn't he?