Thanks so much for the good response to the first chapter! The first chapter was written before the special was broadcast, and so was naively hopeful about the "actual" existence of a 19th-century Sherlock and John, but I've decided to run with the idea regardless. This chapter is a bit of fun based around the Conan Doyle mystery A Study in Scarlet. If you haven't read it, don't worry, there aren't any spoilers; you just need to know that, like most of the original Sherlock Holmes stories, it's told from Dr Watson's point of view.

It had seemed like one of those bizarre dreams, and John found that the matter entirely slipped from his mind until he happened to catch a glimpse of the box on the coffee-table in the lounge. He had to admit to being slightly creeped out: after all, it's not often you find out that another you existed in a different century, living a similar life with another version of your friend. His mind refused to accept the photograph: that insane picture that shouldn't have existed. It blew his mind to see his own face on it, though of course half of it was obscured by a moustache and the shadow of the hat his Victorian self wore.

He wondered what life must have been like. Had he, too, been an army doctor? – there had been wars, so many wars, in the 19th century, in Africa and the Middle East chiefly. It would be a singular coincidence if this other John Watson had served in Afghanistan. And had he been just an ordinary medical doctor back in England?

It was these swirling thought processes that motivated him to think about something else or nothing at all, and so he went back to whatever it was he was doing, after taking the precaution to put the jewellery box out of his sight.

When Sherlock returned that evening – John had no idea where he'd been; he was always disappearing like that these days – Mrs Hudson followed him up the stairs, and let herself into 221B without being invited.

'Ooh, you're both home,' she said, excited. 'I have something to show you both.'

'A book,' said Sherlock dully, without turning round.

'Something to do with our doppelgangers?' John sat up, and Mrs Hudson looked impressed: evidently on this occasion John's deductions were more accurate than Sherlock's. It was then a source of irritation for the detective that John was able to guess more about the matter than he was, because John had an active and lively imagination that was prone to delving into the realms of the fantastical, and the doctor had read more unrealistic books than he had. An unfair advantage, he called it, when the business concerned things that shouldn't be possible.

'Yes,' said Mrs Hudson, lifting a plastic bag onto her lap as she sat down; she drew from it a slim hardback, which she handed to John.

'A Study in Scarlet,' John read from the cover.

Sherlock, who had been leaning back and paying little attention, sat up suddenly.

'By... by John Watson! Mrs Hudson –'

He could not say any more, not quite knowing what was going on, and opened the book to reveal the title page, which again showed him his – his doppelganger's – name and the title of the book, which John thought looked familiar, but which he didn't quite register.

Then he went on to read the first page of the novel or whatever it was ('It's a novel,' Sherlock said then, as if divining his innermost thoughts), and when he had emerged from it his hands were shaking.

'I feel as if this has been set up somehow,' he said indignantly, not wanting to believe what he had read.

Sherlock received the book from him, and read the first page quickly. 'Well, I must say, this John Watson's a better writer than you.'

John glared at him; Sherlock entirely missed his annoyance.

'So in essence,' Sherlock said at length, 'this is the Victorian version of the first mystery we solved together.'

'The Study in Pink,' cried John in realisation.

'And this beginning is how we – our doppelgangers – met.' He scanned the next few pages with his quick flashing eyes. 'It's just like how we met. Your friend Stamford – agreeing to go halves on rooms – Ha!' and his eyes sparkled. '"Holmes was certainly not a difficult man to live with",' he read in mild amusement '– well, certain things might have changed. '"Beating the subjects in the dissecting room with a stick to verify how far bruises may be produced after death" – great minds think alike.'

And with that he closed the book and chuckled.

'This should keep you quiet for a bit,' he said vaguely to John, handing the book back. 'Tell me if you find anything interesting. I'm still looking for a case.'

John could not help but stare as Sherlock revealed himself to be singularly uninterested in the incredible matter, and turned to Mrs Hudson instead. 'So I was a novelist in this past life?'

'It seems,' said Mrs Hudson, 'that Sherlock was a detective, like he is now, and you were the Victorian equivalent of his blogger. From what I've managed to find out, your doppelganger had a few stories published in some popular magazines and also in book form, describing the cases the other John and Sherlock solved together. He was quite successful as well, from what I can see.'

'I knew I should have gone for proper publication,' John said, only a little ruefully.

'You need to work on your writing skills first,' Sherlock murmured from the middle of some daydream.

'What do you know about writing?' and then John's eyes fell on a certain line of A Study in Scarlet. '"Knowledge of Literature – Nil",' he said, grinning suddenly.

'Let me see that,' demanded Sherlock, snatching the book and looking at the relevant section, which was the other John's character description of the other Sherlock. He raised his eyebrows one after the other, until they looked as if they might escape into his hair.

Here Mrs Hudson and John collapsed into laughter, because they had already read this particular part, which was very strongly focused on the weaknesses of this other Holmes. Sherlock appeared to give up on the paragraph quickly – a shame, because it then went on to describe his strengths – and instead turned the page.

Suddenly he laughed. 'Lestrade ought to read this. He's in it as well... "A sallow, rat-faced, dark-eyed fellow" – I'll tell him you said that.'

'I didn't say that,' protested John.

'How did you describe Lestrade in the blog?' asked Sherlock.

'I just said he was a detective inspector. Ordinary. Clever. Nice,' replied John indignantly.

'Coward,' said Sherlock with a grin. 'What is it with normal people? Nobody ever says what they're really thinking.'

'I can't call Lestrade sallow and rat-faced on the blog!' cried John, though a smile was beginning to tickle the corner of his mouth. 'Even though the other Watson wasn't completely exaggerating,' he finished more quietly.

'And here's you calling my articles "ineffable twaddle",' Sherlock informed him, changing the subject as his eyes skated through the book. 'I presume that's the Victorian equivalent of "utter bl**dy nonsense". It would be nice to meet another John who doesn't swear quite so often,' he added. 'It's so unimaginative of you. An entire dictionary filled with words you could use, and you go for the same few every time.'

'I'd sound bl**dy stupid calling things "ineffable twaddle",' John protested.

'On the contrary, you'd sound distinguished and educated,' Sherlock tried to claim.

'Well, that's ineffable twaddle, at any rate,' John chuckled, and they all fell about laughing once again.

Sherlock, though he had been initially uninterested, seemed to find the book more amusing as time went on, and did not let the others so much as look at it until he had finished it a couple of hours later, when he handed it to John with a smile still lingering on his face.

'I'm bored now,' he informed him in an offhand manner.

John could only sigh and settle down to reading, ignoring the strained faces Sherlock was pulling as he tried to resist the urge to let out his boredom in some destructive activity. At length the detective managed to sit back in his chair and relax with his eyes closed and his hands clasped, and when at last John had finished reading A Study in Scarlet – which had singularly fascinated him with its resemblance to their own life and to the first case they had solved (well, a certain resemblance, at any rate) – he caught sight of his friend meditating. He wondered what he could possibly be thinking about, since that whenever Sherlock was bored he had a tendency not to think about anything at all until another case came along.

At last it was time for dinner, and John roused Sherlock from his trance by shouting to him from the kitchen; and over the meal he asked what Sherlock had been thinking about.

'Oh, just wandering around my mind-palace,' he said. 'Some of it looks distinctly Victorian now.'

'Still having fantasies about living in the Victorian era?' John teased him.

'It would be interesting,' replied Sherlock. 'Anyway, I can, now I've set it all up in my mind;' and though he did not elaborate, John could understand exactly what he was talking about. His mind-palace was an extraordinary place, not just for storing information that might come in useful, but for the preservation of memories, and for the construction of fictional worlds in which Sherlock could spend hours wandering. He wondered what his mind-self got up to in the streets of 19th-century London, riding hansoms over cobbles and smoking a pipe and dealing with mysteries that, though they bore certain similarities to those that they solved in the present day, were different if only because of the time in which they were set. Sherlock had a historical playground, and John did not doubt that he would allow himself to play in it.

'Did you put the book on the shelf?' asked John casually when the meal was finished and he was getting up from his seat.

'Yes,' said Sherlock, his eyes twinkling. 'I filed it under I for Ineffable Twaddle.'

'Sherlock!' said John laughing.

'I'm kidding. It's under W for Watson,' Sherlock corrected himself, but his first statement – that rare thing, an intentional joke from Sherlock – still resounded, and the two friends let the kitchen ring with merry laughter.