After their first successful case, Sherlock unravelling a truly bizarre tangle of house-breakings and smashed imported Deltan sculptures, they celebrated over Chinese. It was a cramped, dimly lit back-room restaurant, where the food was cheap, plentiful and absolutely authentic.

"You remembered," John said, grinning happily as he slurped expertly at his soup. It had been one of the first lessons John had taught him, the first time they went on R&R together in Kabul: hole in the wall places well-known by locals but not tourists.

"Of course," Sherlock replied, preening. "This place reminded me of you. You were right, John, when you said London was the greatest city in the world."

John smiled at Sherlock's obvious enthusiasm. Sherlock had never told him of his childhood, but John could guess: born and raised in a laboratory, transferred to Colonel Moriarty's control where he disappeared into the very murky world of overseas special operations – there could not have been time for leisure or exploring.

In fact, John had to wonder how much exposure Sherlock had had to real life and ordinary people, before Mycroft Holmes had, in a move that had used almost all his influence and left Moriarty fuming, transferred Dr John Watson to Sherlock's extremely classified unit.

The wind from the helicopter's rotors whipped up dust and tiny stones and stirred his short, sandy hair, but John had long since grown inured to it; he merely squinted his eyes against the flying grit and jumped out, dragging his med-kit and backpack with him. He crouched down low and sprinted out from under the spinning rotors, waited until the helicopter rose, then lifted his hand in farewell – the pilot tipped him an ironic salute, and then in no time at all was on his way back to the base.

John watched the small craft grow smaller and smaller, receding into the distance, and wondered whether he had made a very great mistake. But the man with the umbrella had given him very little choice –

"Bosnia or Belfast?" a voice drawled, the cut-glass RP accent incongruous out here in the middle of nowhere. But John had heard the crunching of boots on stone and dirt behind him, had been forewarned of the arrival of his new unit.

Still. When he turned, he was not – quite – prepared for what he encountered.

"You said you didn't believe in Queen and country," John mused. "I said it was because you had no concept of England other than as an abstract."

"Well, I've walked down cobbled London streets now," Sherlock said, "and I've taken the tube at rush hour – God, what a horrible experience – and I've eaten late night kebabs and fish and chips with vinegar, wrapped up in newspaper. I've been to a football game, and I've gone out into the wretched countryside, and I've met countless people all crammed together, all different ages and races and walks of life, all of them 'Londoners' or 'English' or 'Man United fans' or –"

John laughed, held up a hand. "Right, right – and what did you find?"

"Most people are idiots," Sherlock drawled, leaning back in his chair. "That holds just as true here and now as it did 300 years ago in the desert."

John pulled a face. "Well, I think that one's going to hold true no matter where or when you are, Sherlock. What else did you learn?"

Sherlock looked at John, kind, honest, patient, hands steady and eyes clear and alert – the first true friend Sherlock had ever had, so ordinary, and so truly unique.

"You're just a man, John," he said, "like any other man, and yet…you're in no way inferior or expendable. Moriarty was wrong. And Mycroft, damn him," he muttered, finally admitting what he had suspected all along, "knew exactly what he was doing when he sent you to me."

The man who had just been delivered to them was a doctor – no, a surgeon, absurdly over-qualified for field operations. But the deep tan, the calloused hands, the confident posture and the adrenaline light in his eyes said it all: this was not a man content to wait behind the lines at the field hospital, waiting for casualties to be airlifted in. This was a man who had run with the Special Forces, got a taste for it, and had no wish to go back.

For all that, he was still only a man. Natural-born, subject to the genetic lottery, with none of Khan's enhanced stamina or intelligence.

To Khan, that made him all the more fascinating.

"Bosnia or Belfast?" he asked, apropos of nothing.

The doctor frowned, fine crow's feet radiating out from his clear, intelligent eyes. "Sorry?"

"I asked where you developed a taste for conflict, Doctor. Was it in Belfast or in Bosnia that you first abandoned the field hospital for the field?"

"Ah." The doctor looked bemused. "Bosnia. Although yes, I was posted to Belfast as well – how did you…?"

"Oh, please," Khan sighed. "You're wearing RAMC patches, but no self-respecting RAMC doctor would find himself assigned to us or even hold himself like you do – you hold yourself like a real soldier, like you've not just seen combat, but actively participated, so somewhere in your career you left the safety of the field hospital behind and have actually gone out into the field. Where have there been British operations big enough to have field hospitals as well as Special Forces deployed in the last – say – fifteen years? Northern Ireland, Bosnia, Afghanistan and Iraq are the most obvious choices. And quite frankly, doctor, I think you drifted away from your RAMC fellows quite a long time ago. Now – am I right?"

Normally, this was the point where the subject of Khan's cutting, almost vicious insight would grow defensive and enraged. This man, this doctor strayed far from his path, did neither – he looked at Khan in bemusement and growing wonder, and he said, quite frankly, "Yes, you're right - that's brilliant."