Returning to her office after a lunchtime meeting, Sarah flicked her phone off silent, checked for texts and frowned. Missed call from - could that possibly be right? It could be a mistake, a chance dialing from his pocket, but that was unlikely with this young man. So careful, so precise - no, if there was a missed call then he must have tried to phone her. Why now after all this time? Her mind raced through a myriad of possibilities, none of them good.

If Sherlock Holmes had tried to phone her then he must be in trouble, one way or the other. She hadn't spoken to him for over a year. They had corresponded for a while. Newsy letters on her side, filled with anecdotes from work and snippets of information that she knew that he would find interesting. Postcards from him, again often containing random pieces of information, more esoteric than hers; about science, about music, occasionally with a brief indication of his well-being. 'More pointless exams passed' or 'Back home for the summer.' As always, she could read more into what he didn't say than what he did say. She read his moods from the neatness of his handwriting, erratic and messy during his ups, careful and beautifully spaced at other times. His lows she read from the weeks or months of silence. At times like these she would often send him a letter of her own, often with the same brief message of her on. 'I'm here, you know. Any time. You just have to pick up the phone.' But he never had, not until now.

Pushing her office door closed, she pressed the redial button and listened as the rings clicked into a default answer phone message. Undeterred she tried again, and this time the phone was picked up on the third ring.

Silence from his end, only the sound of his quiet breathing letting her know that he was there. She could imagine him sitting there, head in hands, wanting to speak but finding himself unable to find the words.

'Hello, stranger,' she said gently. 'How are you?'

She allowed him a few minutes silence before saying, 'Sherlock, it's Sarah. You phoned me, remember? Half an hour ago.'

'So I did,' he replied finally. 'How did you know?'

He sounded dazed, his words slightly slurred. Drunk? She wondered, or worse?

'Missed call on my phone,' she said calmly. 'I was in a meeting, my phone was turned off. So what's happening, are you okay?'

'Sarah?' He sounded lost, unsure. Sherlock was never unsure, not when he was well, as she had come to learn over all those months of caring for him.

'I'm here, Sherlock,' she said calmly, 'Tell me what's going on.'

'I screwed up,' he said quietly, 'and now I don't know what to do.'

She waited for him the explanation that didn't come, and some sixth sense derived from years of working with troubled teenagers made her afraid for him suddenly, conscious of the need to ensure his safety.

'Where are you?' she asked, changing tack.

'I don't know,' he said, and she could hear his relief at being able to concentrate on concrete facts rather than emotion, relieved at least temporarily from the need to explain whatever it was that he was finding so difficult to put into words.

'I walked,' he was saying, 'Along the river.'

'From college? Or are you at home.'

'From college. I couldn't - I had to get out. I walked and I kept walking, and now I don't know where I am.'

'Are you still by the river?' She knew that he wasn't, she could tell from the acoustics that he was in a building, but she needed to get as much information from him as possible.

'In a pub, in a village.' He hesitated slightly, and then sounding slightly ashamed added, 'I don't know where exactly.'

'Thats fine,' Sarah said gently, hiding her concern that Sherlock, who noticed everything, who remembered everything, obviously had no recollection of the route that he had taken. Drugs perhaps? He didn't sound distracted enough to be psychotic. He sounded extremely rational, just - dazed. She wondered exactly how deep this trouble of his went.

'I need you to help me work this out, okay?' she said. 'I need to know where you are.'

'Why?' he whispered, wary now, and sounding more than a little confused.

'So that I can help you, Sherlock,' she said, keeping her voice calm, trying to keep the concern out of it. She didn't want to panic him into hanging up the phone. 'You need to trust me. Do you trust me.'

'Yes,' he whispered.

'Then use that great brain of yours, look around you and see if there's anything that tells you where you are. A menu, a book of matches, anything.'

'Hang on,' he said, and she could hear the rustle of clothing as he got up to investigate. 'The Plough,' he said finally. 'I'm in a pub called The Plough, I don't know where that is though, there's no address,' then with an edge of panic. 'Sarah, don't tell Mycroft, please - just. I couldn't bear it.'

'I need to keep you safe, Sherlock,' she said gently, 'but I promise that I won't tell Mycroft yet, and only if I have to. But I won't lie to you and make promises that I can't keep.'

There was silence for a long time. She wanted more than anything to be able to be there. He sounded so lost, so alone. She wanted to be there with him, to offer what comfort she could, to help work a way through this, whatever 'this' was, but what she needed to do, she knew, was to keep him talking now until she could get him the help he obviously so badly needed.

'Sherlock?' she asked finally, wondering if he had abandoned his phone and walked out of the pub.

'I'm scared, ' he said, hesitantly.

'I can help,' she said, 'let me help. Tell me what's been happening. Have you been drinking?'

He mumbled something in reply, and she realised that she had been right. There was more than alcohol involved here, something that he was ashamed of.

'Sherlock, you can say anything to me, you know that. I'm not hear to judge you, I'm here to help.'

'I screwed up,' came his slightly choked reply. He was struggling not to cry, she realised.

'Its not just alcohol, is it?' she asked. 'You've taken something else. Benzos? What else.'

He was crying in earnest now. She could imagine him sitting there in the pub, his back turned to the room, huddled into a corner, trying not to let anyone see, trying not to attract attention to himself.

'Its okay,' Sarah soothed from the end of the phone. 'its okay, I'm here, I can help you, you're not on your own.'

'I'm out of control,' he whispered, between sobs, 'I can't handle this, I thought that I could, but I can't stop, I can't go back to college, I don't know what to do.'

'You can let me help,' she told him, 'just stay on the phone. Now tell me what you've taken.'

She could tell that he was trying to control himself, trying to get the words out. 'Cocaine? Heroin?' She was deliberately starting at the top of the ladder, assuming that he'd deny it and admit to some lesser class of pharmaceutical agent, but instead he whispered, 'Both, and whatever else I can get my hands on.'

'Do you want to stop?' she asked, realising that she wasn't surprised by this revelation. She had almost been waiting for it. Self-medication in bipolar disorder was common, especially in patients with the severity of illness that he had had. Especially in patients who chose not to take traditional medication. She should have warned him about this. She should have told him of the risk that he took if he tried to control his own neurochemistry on his own. But he was so stubborn, always so stubborn, and he had refused to discuss his non-compliance with medical treatment with her on the one occasion that she had managed to contact him by telephone after he had failed to turn up to his psychiatry appointments at Cambridge. He had been monosyllabic on that occasion, obviously resentful of her interference, and had told her that he hated talking on the phone and preferred letters. He had apologised at the end of the brief conversation though, telling her that he needed to do things his own way and asking her, very politely not to interfere with that. Then, as so often in the past, she had told him that she was there for him, any time. He just had to get in touch. And now he had.

'I need to get away from here,' he told her. 'I need to get my head straight.'

'I can help with that too,' she said calmly. 'Tell me when all of this started.'

As he gradually, hesitantly began to talk, she grabbed a piece of paper, with her spare hand, and holding the phone between her shoulder and her ear, and scribbled a quick note on it, before folding it over, and writing a name on the outside. Then she scribbled a second message of explanation, and as quietly as she could, opened the door to her office, and gave both to one of the junior nurses sitting at the nurses station outside.

The nurse read the explanatory note, nodded and with gratifying speed, took the letter and left the ward to deliver it to its intended recipient.

For anyone who hasn't read The Box, Sarah is the psychiatric nurse who looked after Sherlock while he was an inpatient at Elmhurst as a teenager. After his discharge from there, Mycroft employed her to look after Sherlock at home for six months. So she knows him well, and I like to think that she's one of the few people that he'd trust enough to go to for help.