The case itself had looked deceptively straightforward. A man found dead in the alleyway behind a nightclub, his body half dragged behind the industrial waste bins.

Apart from the fact that it wasn't simple at all. The man was a local businessman, with no police record, no history of substance abuse, and nothing to explain the industrial quantities of cocaine that were found in his inside jacket pocket.

Post-mortem showed the cause of death to be a single stab wound to the chest, piercing the left ventricle of the heart in one quick attack. A professional job, by the look of it, but with absolutely no motive that Sherlock and John could find.

The man had no cocaine or any other drugs in his blood when it was tested. Sherlock had gone through his emails, texts, phone calls, and internet browsing history with a fine toothcomb, examining both his home and work computers for hours, but he appeared to have led an entirely blameless, if slightly boring life.

The cctv cameras leading into the alley had been conveniently disabled, as had the ones at the exit to the nightclub, so there was no cctv footage to assist the search for the killer.

Forensics were still looking for DNA evidence, but so far they too had drawn a blank. There were no footprints in the alley, no fingerprints on the corpse, and no murder weapon, despite Sherlock's extensive search of nearby bins and hedges.

In short it appeared to be the perfect crime, and Sherlock was becoming more than a little frustrated. John had noticed the signs before Kate had. He always became snappy when things weren't going his way, not helped by Anderson's snide remarks earlier, and the fact that he hadn't slept for over thirty six hours. When Sherlock was frustrated, he lashed out at those close to him. John knew this, but he watched Kate's reaction with interest, intrigued to see how she would deal with this side of Sherlock.

The first sarcastic comment she allowed to go unremarked upon. The second also, but then Sherlock flew off the handle when she suggested that the doorman, who John and Sherlock had both questioned the previous evening, might know more than he was letting on. Walking back in from the kitchen where he had been making tea, John missed the majority of Sherlock's explosion, but the words 'irrational' and 'idiotic' were definitely in there.

Kate was looking more than slightly shocked, staring at Sherlock as if he had just grown at second head. Sensing an argument brewing of a scale that he would rather not be a witness to, John muttered something inane about needing to get something from his flat, and scuttled out the door before the fireworks could start.

Kate, however, was not fond of fireworks, or of confrontation. She stared at Sherlock for several minutes after he had turned back to his wall of information, ignoring her entirely, and then silently walked across to collect up her coat from where she had left it on the back of a kitchen chair, and walked quietly towards the door.

She was so angry with Sherlock that she was planning to walk out of the flat without saying a word. But Sherlock turned as she was walking towards the door. Of course he did.

'Where are you going?' he asked, sounding surprised.

'Home,' Kate said. 'Since I'm obviously an idiot and not helping, I'm going home; to sleep, and to not be shouted at.'

Sherlock buried his hand in his curls in his characteristic gesture of frustration. 'I didn't say that you were an idiot,' he said, slowly, with poorly concealed irritation. He needed to concentrate, and Kate wasn't helping that.

'No, but you implied it.

Sherlock stared at her, his expression softening as he stopped seeing her as another John, there to help him work, and saw her for what she was. Kate, his Kate, and he wanted her to stay. 'But I don't want you to go,' he said, suddenly childlike in his simplicity. 'I need you here.'

Then when she shook her head, 'I'm sorry?' he asked questioningly, as if this could wipe away whatever it was that had upset Kate. Emotion. It was so complicated. It interfered with logic. It interfered with the work.

'No, you're not sorry,' Kate said, with a sigh 'You're just saying that to get me to stay. I'm sorry Sherlock, I can't. I have to go home.'

Her anger had melted away as quickly as it had come. She couldn't explain this to him now, but equally she couldn't stay.

Reaching up, she swiftly kissed him on the cheek. 'I will be many things to you, but I won't be your punch bag,' she said quietly, allowing her hand to linger against his cheek for a moment. 'I'm sorry, I can't do that. If you need someone to shout at, then you'll have to find someone else.'

He still looked confused as he caught her gaze and held it, 'But I don't want anybody else,' he said quietly.

'I know, so we're going to have to find some way to work this out. But for now, I'm going home, not because I'm angry, but because I can't be here at the moment. But I'll phone you in the morning, and I'll come over after work tomorrow if you're in a better mood.'

'Stay,' he murmured as he pulled her into a hug, strangely relieved when her arms came up to hold him close and she let her head rest on his shoulder for a moment.

'I can't,' she said into his shoulder, 'but this doesn't mean that I don't love you. It just means that I need to be away from here.' and kissing him quickly, she turned and walked out of the flat, closing the door behind her. Sherlock stood where she had left him, bemused and frustrated.

At the bottom of the stairs, she hesitated and knocked on John's door. He opened it almost instantly.

'Okay?' he asked her.

'Yes, fine. I'm going home, John, and I don't think that he has any concept why. Just - keep an eye on him for me will you? You know what he's like. Keep him working and don't let him come after me, he needs to stay focused.'

John nodded. 'Its not just you, you know. He shouts at everyone when he's working.'

'I know, but everyone else can put up with it, I can't. I've told him that.'

'Are you going to be okay getting home?'

'Yes I'll get a cab, its fine. Oh and John,' she turned with her hand on the street door. 'When he's more receptive, can you get him to look at the doorman again? I'm sure that he knows more than he's letting on.'

John nodded. 'Okay,' then impulsively he gave Kate a quick hug. 'Kate you're an angel for putting up with him, I hope you know that.'

She shook her head. 'I'm not doing it very well tonight, though, am I. Tonight I'm running away, but I'll be back tomorrow.'