There had been a lot of clunking in 221 Baker Street that morning, and John was beginning to become worried. He had heard Mrs Hudson go upstairs – she had asked on her way up whether he wanted a cup of tea (he had declined politely, but was beginning to wish he hadn't). Then she had proceeded to stay upstairs all morning, and made this racket whilst doing so. He decided she must be doing some form of DIY, and he rather hoped it wasn't in his bedroom, considering the amount of murmured curses that came to him through the ceiling. No – if he stood directly underneath the noises he found himself beneath the upstairs landing, where, from what he could hear, he guessed that she was doing something to the ceiling.

Unusually, Sherlock was out that day and he was in, because Sherlock had been made to visit his parents. (Made was quite the right word, because he didn't like going to see his parents, but even Mycroft had insisted somewhat smugly – smugly, that is, until Sherlock had informed him that he was invited too.) No doubt Sherlock would have guessed in an instant what Mrs H was up to. But he wasn't there, and John was getting curious, so he got up from his armchair and decided to go and investigate.

When he clambered onto the upstairs landing just outside his bedroom, he found Mrs Hudson balanced unsteadily on a ladder and trying, it seemed, to drill through the ceiling, which wasn't quite what he had expected. She didn't notice him at first, and so he waited until she switched the drill off for a moment before greeting her and asking what was going on.

'Oh, hello, John.' Mrs Hudson turned round and caught sight of him through a pair of enormous goggles (John didn't ask where she had got them from). On taking them off she realised that she had managed to shower the poor doctor with bits of paint and plaster, and that he now resembled an explosion in a flour mill. 'Oh, gosh, sorry! Hang on, let me brush that off…' And she swiped vigorously at him, brushing the white flaky bits onto the fragments of the Times that covered the landing carpet.

'Mrs Hudson…' John began. 'What… on Earth… are you up to?'

'Ah!' And Mrs Hudson looked excited. She bent over and picked up a piece of paper. 'I found these the other day. A sketch of this house. A plan, I mean.'

John looked, and saw that the paper showed an old pencil drawing of each floor of 221 Baker Street. It was fascinating enough, but he couldn't work out why that merited drilling through the ceiling.

'And look.' Mrs Hudson gently flipped the paper over. 'Apparently 221 used to have – still has – a loft. They call it an attic on the plan, but I think it was a loft. The hatch had been painted over and hidden, but I've managed to uncover it.' She pointed to the ceiling, where there was now a lack of paint and the outline of a loft hatch. 'The hatch had become stuck and so I'm trying to get through it.'

John raised one eyebrow and wondered why Mrs Hudson was excited about a loft. 'But what are you expecting to find up there? Treasure?'

'Well, maybe,' said Mrs Hudson enigmatically. 'I've heard that previous occupants of this house have included some wealthy and some interesting gentlemen and ladies. They told me when I bought it that it had a fascinating history.'

Now John was a little more interested. 'I can help you, if you like. See if I can get the hatch open now.'

Mrs Hudson nodded, and stepped to one side to let him scale the ladder. John leant his shoulders against the hatch, his head bowed and grazing the ceiling, and pushed upwards; then he nearly fell backwards as the hatch came unstuck and flew into the loft.

'I'm in,' he said a little pointlessly. 'D'you want to grab a torch? It's dark up here and I don't imagine there's a light switch.'

A few minutes later they rendezvoused on the landing, Mrs Hudson clutching a bright torch and John in scruffy clothes that could get as dusty as they liked. Then they both scaled the ladder and clambered into the loft.

It was a dark and drafty little space, smelling of dust but not damp; and they were surprised to find a number of items: a couple of boxes containing a smart set of silverware and crockery that must have been at least a hundred years old; an ancient slice of wedding cake that had 1920 on the label, alongside two names that John couldn't quite make out; and a curious little wood and metal chest that didn't seem to want to open, and so he took it down the ladder into the proper light, Mrs Hudson following with the silverware, which she had taken a fancy to (but which needed dusting before she used it).

It resembled a large jewellery box, but when John shook it a little he could hear little rattling inside it, though it seemed that there might be papers in there. In the light he saw that there was a small key attached to the underside of the box, and so he took it downstairs into the living room of 221B and made to open it; that was when Sherlock returned, and looked curiously at the box on the coffee-table.

'Late Victorian, belonging to a woman who liked to think she was richer than she was,' he said vaguely. 'Did Mrs Hudson find it in the loft?'

'You knew she'd been in the loft?' John asked in surprise.

'No,' Sherlock replied. 'I saw that you were covered in ceiling-plaster from upstairs and deduced that Mrs Hudson had been looking at the house plans that had been in her possession. I knew there was a loft. I didn't think there would be anything of interest in it.'

His disjointed statements only confused John more; after he had spoken however John said: 'But there is this. This is quite interesting.'

'A jewellery box.'

'It has papers in.'

'Letters, then. If it's a woman's. Women keep letters. Photographs as well, perhaps.'

And Sherlock, disinterested, went to sit in his armchair, clutching a copy of the day's newspaper, which he read in a desultory fashion: John knew that he often did this when there was a lack of cases for him to solve, just in case he might chance upon some interesting morsel that he had not heard about, or, in the agony aunt column, a problem that was more than it seemed at first glance; sometimes he read that column just to amuse himself by deducing things about the people who wrote in.

Just then there was a click as John got into the box, and the lid swung upon, unleashing a cloud of dust that had sat on the top for however many years – more than a hundred, if "late Victorian" was correct. He peered into the box, with Mrs Hudson looking over his shoulder, and then furrowed his brow as he chanced upon the first items in the box: a slender and richly decorated, though somewhat blackened, pipe, and a small pot of tobacco that was half empty.

John passed the pipe first to Mrs Hudson, who was now almost overexcited, and then to Sherlock, who looked mildly distracted for a moment before saying: 'A regular, perhaps too regular, smoker. He used various varieties of tobacco – the one in that pot is just one of them. That one is a common Victorian pipe tobacco. There are residues of rarer, even foreign, ones on the pipe. The blackening shows that he used it often. It was perhaps his only pipe – he was either attached to it, or his finances were such that he could not risk buying a new one.'

'So why did he put it in a box in the loft?' asked John at length.

'I don't think he did. I keep telling you – that's a woman's jewellery box. And women tend to be sentimental. A wife, a daughter or a friend, I imagine. I expect this gentleman died, and his relatives and friends could not bear to get rid of his possessions, but could not face them either.'

He studied the pipe for a moment more, and then handed it back to John, before saying unexpectedly, 'It's rather a nice pipe. I had fantasies of being a Victorian gentleman and smoking a pipe a bit like that.'

'When you were little, you mean?' asked Mrs Hudson.

'When you had given up wanting to be a pirate?' asked John.

'When I was at university,' Sherlock replied vaguely, before settling back down in his seat.

'There might be clues in here as to who this gentleman was,' John commented, and lifted the tobacco-pot carefully from the box to reveal the layer of assorted papers. The first was a sheet that was to protect the others from the tobacco-ash. Underneath it was a small sepia photograph that was not of great quality, but which, when John lifted it to the light, revealed itself to be singularly spectacular.

'Good God,' John muttered involuntarily, looking astonished. His hands began to shake as he flipped the photograph over to reveal, in spidery handwriting that he could have sworn he recognised, names that he knew very well indeed:

Sherlock Holmes and Dr John Watson in 221B, Baker Street.

'Good God,' John said again.

Now Sherlock was more than interested; he took the photo a little roughly from his friend's hands, and inspected it. It appeared to show two Victorian gentlemen in front of the fireplace in the very room they were in: two Victorian gentlemen who, had they not been in the dress of the time, would have been almost-doubles of the two men who were currently in that same room. The one on the left was small and smiling and wore a moustache and a somewhat mismatched suit (the Victorian equivalent of John's odd taste in jumpers, Sherlock thought without voicing it); the one on the right was tall and stern and topped by a deerstalker not unlike the one Sherlock loved to hate, and though the image was grainy and unclear there was no mistaking the high forehead and distinctive cheekbones.

'It seems we have historical doppelgangers,' he said, not wanting to show that he was as bewildered as John.

'Have you seen their names yet?' asked John in a shaky voice.

Sherlock flipped the photo over, but did not need to, as he had in his heart already guessed. These Victorian gentlemen were somehow them… just in a different time.

'So the pipe…' John began.

'…was one of ours in a past life,' Sherlock finished.

'What are you two talking about?' Mrs Hudson asked then, holding out a hand for the photograph. When she looked upon it her face showed about a hundred expressions all at once, and John and Sherlock would have laughed if they weren't feeling exactly the same way.

'I hope it was mine,' said Sherlock with a slight smile, picking up the pipe and trying to imagine himself smoking it. 'I wonder if it would still light. It's a little blocked, but…'

'Oh, don't, Sherlock dear,' said Mrs Hudson once she had recovered her breath. 'You might break it.'

'I wouldn't have chosen a pipe so fragile,' Sherlock countered a little enigmatically, and went for a match. Whilst he was doing so, John looked at the rest of the papers. Some were just sheets that had signatures on – signatures that were almost unintelligible but which he just knew to be Sherlock's doppelganger's. Just as a dark cloud of musty tobacco smoke began to fill the room – as well as a small sigh of pleasure from within it that John had rarely heard escape Sherlock's mouth – he chanced upon the last paper in the box. This one was another of Sherlock's predictions: a letter, and one that was written in a hand that John knew on sight, for though it wasn't his own – it somehow was.

' "My dear Holmes", ' he began to read, and then his voice faltered as he read through the rest of the letter. It was short and stumbling, yet conveyed a deep sadness that left John almost in tears.

'What is it, John dear?' asked Mrs Hudson.

'A letter,' replied John quietly. 'A letter… from the other John Watson to the other Sherlock. I think… I think the other Sherlock… died. He was the man who passed away… And John – the other John – wrote this to him even though he would never…'

Sherlock, at this, started and looked up.

'The thing is…' John swallowed and looked away for a moment. 'It's almost exactly what I would write if…' He stopped himself short. Though his head was bowed, he could feel Sherlock's piercing eyes on him, and Mrs Hudson's sympathetic gaze. The letter fell from his hands and he did not continue.

'How did he die?' asked Sherlock quickly, in a sudden burst that sounded almost akin to panic.

'I don't know,' John murmured. 'It said something about him passing away at…' He squinted at the letter. 'I couldn't quite make it –' His eyebrows shot up towards his hairline. '– out.' He looked up at Sherlock. 'That's got to be a coincidence.'

'What has?'

'He died at Reichenbach. Like the falls in that painting that you've just – how many Reichenbachs are there?'

'I only know of that one,' said Sherlock quietly.

'It must be coincidence,' said John.

'Yes,' muttered Sherlock, before turning so he didn't have to look his friend in the eye.

There was a silence that was broken by Mrs Hudson asking if they could find out who the woman was whom the box belonged to. John nodded, his train of thought dissolving, and began to inspect the box. At last he found a name inscribed into the wood, another name he knew: Martha Hudson.

'Apparently it was yours,' he said, and handed the box to Mrs Hudson.

There was another awkward silence.

'So in the late Victorian era,' began Sherlock, 'this house was inhabited by another one of each of us – our historical doppelgangers. This box was compiled by Mrs Hudson after the other Sherlock had died and presumably John had left in his grief.' His eyes sparked just a little. 'The other John was a doctor, and I imagine the other Sherlock was a detective.'

'You can't know that,' noted John.

'It's the only thing I'd ever be,' countered Sherlock.

'It's very intriguing,' said Mrs Hudson then, who had begun to smile indulgently at the thought of their alter egos existing in this very house, more than a hundred years ago. 'I wish we could meet them.'

'Mm. And I wonder if we could find out more about them,' John replied. He turned to Sherlock. 'Are you interested yet?'

'No,' said Sherlock at once.

'What?'

'Well, there's no mystery involved, is there? We've found out all we need to about them, and anything we don't know we can fill in easily, because – well, they're us.' His eyes twinkled. 'Find me a case. I'm bored.'

And John and Mrs Hudson burst out laughing at this typical Sherlock-comment, but at the same time decided that their investigations into their historical doppelgangers weren't quite over yet.