Oh God, she was there already. That shy smile, that light touch of mascara that made her look so damned – what was the word? He supposed that lesser mortals than him might call it flirtatious – he just thought it was a bit annoying. Molly Hooper was in his lab, and she was waiting for him.

'Results? Thank you,' Sherlock said briskly as he walked in and removed the sheet of paper from her hands.

Molly's lower lip pouted a little. Sherlock didn't notice. She had to grab his arm to get his attention; even then he looked exasperated to have been interrupted.

'Sherlock,' she said. 'Sherlock, this is serious. You have to come down with me. Come and look at that corpse.'

'I've looked at it enough,' Sherlock murmured, a little darkly.

'Sherlock, it's weird. Everyone's been talking about it.'

'Talking about what?'

'The burn mark across his chest.'

'The – what?' Sherlock's fingers slid from the microscope he had been adjusting. Something sparked in his eyes.

'You didn't turn the corpse over when you were at the crime scene, did you?'

'Anderson wouldn't let me touch it.'

Molly nibbled her lip. 'I'll show you. Come on.'

It was an ugly wound, that much could be said. Almost perfectly circular, but with jagged, seared edges, it spread across the man's chest and seemed to pierce the skin; it was as if he had been set aflame from the inside. Sherlock regarded it with a mixture of concealed horror and morbid interest. It irritated him that he hadn't been able to see this the previous day. Damn Anderson.

'Well?' he said.

'Well what?' asked Molly.

'Was it this that killed him, or the blow to the back of the head?'

'Both... or perhaps neither.'

Sherlock narrowed his eyes.

'Some have suggested it was a combination of the two, because it seems to be a common opinion of all who have seen it that, though both are serious injuries, neither should have killed him – not as quickly as they did, anyway.'

'Are there any other wounds?'

'None.'

'No evidence of poisoning? – No, there wouldn't be,' Sherlock added, almost to himself. 'What could have made a burn like this?'

'That's the thing – we don't know.'

Sherlock straightened, and began to pace around the morgue. The corpse seemed to mock him still, lying there as the centrepiece to what seemed at present like an unsolvable mystery. A blow to the back of the head might have been a fairly plausible solution, but this burn mark had complicated matters immensely. And they hadn't even started on finding out who was behind all this.

'I'm going home to think,' Sherlock said then, and, without a further word, he pulled on his coat and scarf and left.

He didn't go home. He didn't go anywhere close. He went for a brisk walk around London, pacing the streets without registering a single one of them, his footsteps quick and irregular, his eyes ever turned towards the pavement. Thoughts bubbled in his mind and threatened to spill out; his lips moved rapidly, forming words in silence; he was utterly perplexed.

At last, when he was beginning to tire of walking, and when he found he had come to a familiar sight, he decided to stop and see if Mrs Hudson was in. If nothing else, she would provide him with a much-needed cup of tea.

221A Baker Street was something of a refuge now for Sherlock. He liked the smell of tea and fresh flowers, he liked the minimalistic but tasteful decor, he had to admit that he liked the fuss that Mrs Hudson made of him. He rather preferred 221B, but this flat would have to do for the moment.

This time she greeted him even more eagerly than usual, ushered him into the little kitchen and said, without preamble:

'Merlin's interested in your offer of going halves. You're just in time – I said I would meet him at eleven.' Sherlock glanced at the clock – it was just gone ten to. 'I rang your mobile but you didn't pick up.' She frowned a little accusingly at him.

'I didn't notice it ringing,' Sherlock said vaguely.

'Well, I was going to ask you if you could come over, but you're here anyway, so it doesn't matter.' She began to busy herself with making tea.

The tea was ready, and Sherlock had just taken his first sip, when there was a knock at the door. Mrs Hudson went to answer it, and, after greeting whoever was behind it, brought in a young man who was evidently Merlin Ealdor.

He was a tall gangly fellow, with difficult dark hair and an easy-going sort of smile; he greeted Sherlock in a friendly manner, and thanked Mrs Hudson profusely for the tea that she poured him. Then he looked straight towards the detective, curious as to the character of his potential flat-mate.

Sherlock, for perhaps the first time in his life, was astonished by the man's eyes.

He had made a quick analysis of this young man without seeing directly into those intriguing orbs, guessing him to be intelligent and a little scholarly with a wicked sense of humour, among other things. Yet one's eyes can often be the greatest indicator of true personality, and here were eyes unlike any that Sherlock had seen before.

On the surface they were bright, clever, perhaps slightly naïve. Within they seemed to exude a deep wisdom such as a man of his age should not have possessed; they were almost haggard, as if he had seen far too much – these were certainly not the eyes of an innocent man. Sherlock had to look away, for he was perfectly confounded by them. It was a sensation he had never felt before, and a sensation he found he didn't like in the slightest. Already the man seemed as much of a mystery as the murder he had been investigating.

Ignoring all the thoughts that sprang to mind, Sherlock merely shook his hand and introduced himself. Merlin returned the gesture, grinning almost inanely, as if it was his natural expression. It probably was. People with that much hiding behind their eyes had a tendency towards a clumsily childish outward appearance.

'You're a detective, I hear,' Merlin continued. 'That must be fascinating...'

'Yes,' said Sherlock curtly. He didn't much like small talk.

'Well, it's nice to hear you're on the right side of the law,' Merlin chuckled. 'I'm studying. Postgraduate, you know.'

'English literature,' Sherlock murmured.

'It was the only subject left,' Merlin said, a little cryptically. 'Anyway, I'll try not to tread on your toes. I read most of the time. Sometimes I talk to myself, but –' He halted himself, as if he had said too much.

'The violin,' said Sherlock then, starting from his thoughts. 'I play the violin. I hope you don't mind.'

'Oh, I like the violin,' Merlin said. 'Played well, anyway.'

'Good,' said Sherlock. 'And my sleep schedule is a bit haphazard, and sometimes I look like I'm asleep but I'm not – that is, I can live in my thoughts for days on end – will it trouble you not to disturb me if I am like that?'

Merlin looked a little surprised, but shook his head.

'Oh, our Sherlock's a bit of an oddball. I think I mentioned,' Mrs Hudson said.

Merlin laughed. 'So am I, if I'm honest. Maybe we'll get on.' He shrugged.

'Is that a deal then?' asked Sherlock, holding out his hand.

'Flat-mates,' Merlin said, shaking.

'Flat-mates,' Sherlock agreed.