Dropping to his knees beside Sherlock, John felt an initial jolt of panic, as he always did when facing these situations outside work, but it took less than thirty seconds for his training to kick in.

He shook Sherlock gently by the shoulders, calling his name, and when that failed to provoke a reaction, shouted down the stairs for Mrs Hudson.

Sherlock was lying on his back, his breathing slow and slightly noisy, his airway partially obstructed. John tipped his head back, fingers lifting his chin, to improve it, then dropped his ear to Sherlock's chest to listen to his breathing. Sherlock was breathing at a lazy eight breaths a minute. Too slow. He shouted for Mrs Hudson again, just as she emerged from the door at the top of the stairs, rubbing at her hip.

'Sorry, John, this hip of mine. Can't you write me up for anything stronger?' Then she stopped dead, hand flying to her mouth as she saw Sherlock.

'Call an ambulance, will you?' John said, digging his mobile phone out of his pocket and handing it to her. Give it to me when they put you through to ambulance control.'

'What happened?'

'No time for that now. Just call, will you?' John said, as he checked Sherlocks' pulse at the neck. It was a little slow, but John found that reassuring. If he had speed-balled, combining amphetamines and heroin, then his pulse would have been fast. A slow pulse and a slow respiratory rate together pointed to a pure opiate overdose, far easier to treat. He pulled up Sherlock's eyelids to check. The pupils were tiny, pinpoint, confirming the diagnosis.

'You stupid, stupid bastard,' John murmured, just as Mrs H put the phone to his ear. 'Sorry, I wasn't talking to you,' John said to the call handler. 'This is Dr John Watson, I'm a GP. I need a blue light ambulance to 221b Baker Street. I have an unconscious 37 year old man with a heroin overdose and respiratory depression.'

'Are you with the patient?' asked the call handler, reciting from their screen prompts, John knew. He tried not to get irritated. He knew they had to ask but it was ridiculously frustrating being asked the inane questions when all he wanted was to get Sherlock to hospital ASAP.

'Yes, I'm with the patient. No, he's not conscious, and yes he is breathing but barely,' he told her, pre-empting the triad of questions.

'I'll have a crew with you as quickly as possible, the call handler told him.

John just hoped it was soon enough. Sherlock's breathing was much too shallow for his liking, but without any kit there was little that he could do. And Sherlock's lips were still pink, as were his fingers, with no hint of a blue tinge. He must be oxygenating well enough to not be cyanosed, so saturations above - what - 85%? That would have to do for now.

The ambulance seemed to take forever, but in reality, John suspected that it was well within the eight minutes target time. He had sent Mrs Hudson downstairs as lookout to wave them in the right direction, and also, if he was honest with himself, to stop her flapping over Sherlock. While she was gone, he quietly remove the syringe and tourniquet that had been tossed so casually on the floor, and hid them in a biscuit tin in the kitchen. He doubted that the paramedics would blab to the press but no point in giving them further ammunition if they did.

He handed over to the ambulance crew quickly, as they slid an oxygen mask over Sherlock's face, and a pulse oximeter onto his finger. John was gratified to see his oxygen saturations slowly climbing from the initial 87% as the oxygen did its job. He offered to cannulate Sherlock himself, deliberately picking his right arm, knowing that being right-handed, Sherlock would have been injecting into his left in preference. He had far more chance of finding a useable vein on the right.

He found a vein easily in the back of Sherlock's hand, but his upper arms on both sides were a battle zone. No wonder he had stuck to long-sleeved shirts and tops in John's presence for a while. But his hands would have been too obvious, he had known that John would have noticed injection marks on those.

The cannula slid home with a satisfying flash-back of blood and John injected the first dose of naloxone before securing it with the tape and dressing, then attached a bag of saline. Sherlock's blood pressure was on the low side, not dangerously low, but a little fluid wouldn't hurt. And then they were off, carrying Sherlock down the stairs on the ambulance trolley, John marvelling as he always did at the physical strength of the paramedics. Sherlock was relatively light compared to some of their patients, he knew, but still, carrying a 65kg man down a flight of relatively steep stairs was no mean feat. No wonder they didn't allow pregnant paramedics out on the road. The physical demands of the job were too great.

By the time they got Sherlock into the back of the ambulance, he was groaning slightly, but his pupils when John checked were still tiny. His oxygen saturations were now a steady 96% on oxygen, and John administered another dose of naloxone - intramuscularly this time, into the deltoid muscle in his arm. It was a calculated risk. Intramuscular naloxone worked slower and lasted longer. If he gave Sherlock another intravenous dose then he might well wake up entirely, pull off his oxygen, and refuse to go to hospital. If he did that then he was highly likely to collapse again later on as the naloxone would wear off before the heroin would. By giving him an intramuscular dose, John was ensuring that even if he did do a bunk later, he should stay safe - and breathing, and he was more likely to get Sherlock to hospital and to ensure that he got the help that he needed.

The sirens started up as they sped through the streets towards St Mary's, cars climbing pavements to get out of their way and enabling them to plough a gratifyingly speedy path through the congested London streets. Arriving at A&E, John was relieved to be able to hand over Sherlock's care to the consultant working in the Resus Room that day, and to take a metaphorical back seat. Sherlock was rapidly assessed and then hooked up to the naloxone infusion that he needed. John couldn't help wondering how many times Mycroft had been in exactly this position; watching the medical staff take care of Sherlock, aware of all of the preconceptions that they would have about him and yet equally unable to deny any of them.

Several hours later, Sherlock was moved to a ward, John having pulled in a few favours to get him a side room. In theory, he should be somewhere in direct eye line from the nurses station to allow him to be observed, but John had offered to do the observation himself in return for the privacy the private room afforded.

He sat there, watching Sherlock sleep and wondered where the hell they went from here. That Sherlock had a significant drug problem was obvious; that he was in complete denial about it was unfortunately equally obvious. But surely even Sherlock would be forced to admit that when you ended up in hospital from an overdose, then it had gone beyond 'using' to 'abusing' drugs?

'Unless he wanted to overdose,' said that uncomfortable little voice in the back of John's head. 'Unless he was trying to kill himself and you're just assuming that it was accidental.'

And John Watson, being the sensible, logical human being that he was, tried very hard to push that thought aside. Of course Sherlock hadn't been trying to kill himself, Because why would he? He was back, he was safe, he was shortly going to receive a Royal Pardon if Mycroft had anything to do with it. And he had a case to work on - a massive case, potentially one of the biggest if his career. Why would he want to kill himself? And yet that nagging thought just wouldn't remain silent. And John couldn't help feeling that he was somehow at least partly to blame.