"I like what you've done with the place."

It couldn't be.

Sherlock gingerly uncurled himself from the foetal position he now found himself in, knees almost wedged under his chin in his chair, and craned his neck to look for her. And there she was, perching on the windowsill that overlooked Baker Street, suppressing a smile, her eyes alight with mischief.

"M-Mary," he stammered, feeling for the floor with his feet.

"I mean, the flat is still a pigsty, but I love this new chair," she continued, gesturing, as though she'd just come by for a cup of tea. "Didn't think yellow was really your colour, but perhaps it's not really meant for you...?"

Sherlock tested his legs, swaying slightly as he got to his feet. Taking incremental steps towards her, he had to remind himself that this wasn't real, that she wasn't actually there, however badly he wished it were otherwise. But it felt vivid and palpable, and he felt an overwhelming rush of emotion.

Mary was watching him with affectionate amusement.

"You look like a baby giraffe, Sherlock," she grinned. "Either that or you've decided to go all-out on the Christmas sherries."

If this was in his head, if he was no more physically there than she was, then what was to stop him from hugging her? He tested his theory, surprising her as he took the last few steps between them quickly and engulfed her in his arms.

"Easy, tiger," she chuckled, rubbing his back. "You know, this is typical of you, you silly sod; I have to wait until I'm dead to get a proper hug."

Her smile, as always, was disarming and infectious, and Sherlock couldn't help himself.

"I…I know this isn't real, Mary, but I…it's so good to see you again," he said, haltingly. "I never…even after everything that's happened tonight, I didn't expect…this."

"What? You think I'd pass up an opportunity to come and haunt your arse from beyond the grave?" Mary grinned. "If anyone has earned the right to do that, I reckon it's me."

He nodded, aware that he was suddenly remembering the shimmering light and eerie sounds from the aquarium that had dominated his nightmares in the months after Mary was gone.

"Oh, Sherlock, there's no need to look so melancholy," Mary said, nudging him. "It's Christmas, remember? Come on, come over and sit with me."

She led him across to the sofa, sitting down and patting the cushion beside him.

"This reeks like a student house, by the way," she said, screwing up her nose. "What have you been doing on this sofa?"

Sherlock frowned, defensively.

"Not that," he told her. "If that's what you're implying."

Mary gave a short snigger.

"Yeah, chance would be a fine thing, eh?"

She shifted around to face him a bit better.

"You look bloody awful, Sherlock Holmes," she said, shaking her head slightly. "Such a waste of that gorgeous little mush of yours."

She patted his cheek in a manner of sisterly teasing. God, he missed this.

"It's not been a great year," he replied, sighing.

"Tell me about it, mate," Mary said with a sly smile.

"Sorry, I didn't mean-" – he took another breath – "It's just…everything fell apart when you…"

"Died," Mary put in. "It's okay, you can say it. I'm completely aware of my deadness - I'm not having some kind of Bruce Willis Sixth Sense delusion here."

"What?" Sherlock asked, puzzled.

Mary rolled her eyes; shook her head to tell him never mind. That used to mean that another trivial pop culture reference had just zoomed over his head. She gestured for him to go on.

"But I did what you asked me," he continued. "I went to hell – right to hell - and I saved John. Well…it was sort of a mutual saving process."

Mary was looking at him thoughtfully.

"Sherlock, you know when I said that, I meant the village in Norway, right? Hell?"

Sherlock blinked, eyes shooting up to look at her. She held his gaze earnestly for a long moment, then pinched her lips together in a smirk.

"Sorry - I'm messing with you," she grinned. "Though you can't really blame me – I really, really miss seeing your face when I do that."

Sherlock let out a bark of laughter, leaning over to rest his elbows on his knees, and looking sideways at Mary. She was still grinning at him, but as they looked at each other, her expression began to shift into one more serious. She nudged his thigh with the back of her hand.

"I'm grateful for what you did for John, Sherlock," she said. "That man…well, he's a lot more complicated than he might seem. He's stubborn, and he blames himself for everything – even the things he seems to be blaming other people for - and I knew he wouldn't accept help of the conventional kind."

She leaned a little closer.

"'Course, it would have been better if you hadn't nearly killed yourself in the process," she added. "Two of us being dead wouldn't have been much bloody help to anyone."

Sherlock nodded, running a hand through his now-unkempt hair.

"I…I wish you could see Rosie," he started, carefully. "See her growing up. She changes so quickly."

"Mm," Mary nodded. "They do that. I know…I know John will probably never understand my decision, but for what it's worth…I know Rosie is in good hands. That I left her with people who will do all they can to give her a good life."

He immediately thought of Molly again, started to drift to the small room in his Mind Palace that he had specifically set up for memories of Molly with their goddaughter. It was a little annex from the main room he had dedicated to Molly herself, which in itself had many extensions built on over the years. Molly with Rosie on her hip, making the baby some lunch; Molly singing softly to Rosie, completely unselfconscious, despite him being in the room; Molly reading Rosie a picture book – far too advanced for her – about great women in science.

"I…I try to be a good godfather," he said, hearing his voice waver. "I'm still learning. But I do love her. Very much."

Mary smiled, warmly.

"I know you do," she said softly. "I always knew you would. How's Molly, by the way?"

The sudden question took Sherlock by surprise, and he strongly suspected it was intentional – he knew Mary Watson's methods. He tried to recover from the conversational ambush, sitting more upright, clearing his throat.

"She's well," he replied, evenly.

"Still putting up with you, then?"

Sherlock made a noncommittal humming noise, which he hoped would suffice.

"Oh," Mary replied, arching an eyebrow. "It's like that, is it?"

"Molly and I have reached an understanding," Sherlock said slowly.

"'Reached an understanding'," Mary repeated, doing her best Sherlock Holmes impersonation. "Are we in a Jane Austen novel now?"

Sherlock rolled his eyes, feeling his cheeks start to warm to an uncomfortable degree.

"We're fine. It's fine, it's all-"

"Fine?" Mary interjected. "Well, that's good to hear. You've clearly been very grown-up about all this, Sherlock. Not burying your head in the sand at all. Not running away or trying to hide from the obvious. Just facing it all in your best big-boy pants."

Sherlock scowled.

"Mary, far be it from me to speak ill of the dead," he said. "But shut up."

"All right!" Mary replied, throwing up her arms in surrender. "I can take a hint, Mr Touchy. Shall we get on to why I'm here, then?"

"What? Taking the piss isn't enough for you?"

Mary poked his leg in admonishment.

"Don't worry, I'll keep it brief," she said. "I know you've had a long evening. And you're going to feel like shit when you finally do wake up, you daft bugger. Am I allowed to ask how you ended up so distracted that you gassed yourself?"

"No!" Sherlock replied. God, he'd forgotten that it was like a war of attrition with her sometimes; if she didn't get what she wanted first time, Mary Watson would just grind you down – smiling endearingly while she did it.

"Suit yourself," she shrugged. "But you need to come with me. Come on, on your feet, Lanky!"

Mary took his hand and yanked him off the sofa, pausing to take a look at him, to rearrange his dishevelled curls, to brush something from his jacket lapel. Once she was satisfied, she took his hand in hers again and led him to the living room door. Sherlock felt his heart pounding, acutely aware that he was about to be shown a vision, a projection of – presumably – his future. Or at least a future that a fictional Mary would choose to show him. As long as she wasn't about to lead him straight to his headstone, then, he decided, it was probably going to be fine.

"We should have gone through the wardrobe," Mary said, when they emerged on the other side of the door. "Might have led to Narnia. Although your wardrobe would probably lead somewhere far less wholesome, and I probably don't want to see you've got hidden in there anyway."

Sherlock was about to protest that there was nothing whatsoever untoward in his wardrobe when he realised that he was in an unfamiliar house, his back to the closed front door. He began to take in his surroundings – Victorian townhouse, high ceilings, airy and brightly lit. Stained glass in the front door cast colourful puddles of light on the stripped wood floor. No photographs on the walls of the hallway, but some prints and paintings instead, slightly quirky, at odds with the grand home, but in a way Sherlock found strangely pleasing.

The hallway was hung with paper decorations, and Christmas cards had been pegged to gold ribbons that hung from the picture rail. At the bottom of the stairs, a child's bobble hat had been stuck on the post at the end of the banister, and a pair of mittens on strings hung over the rail.

He could hear voices coming from another room, further into the house, mingling with each other in a way that suggested a get-together.

Was this John's house? The hat and gloves could conceivably be Rosie's, although the artwork didn't look to his taste.

Just then, he saw movement out of the corner of his eye and he turned just in time to see a cat pad silently down the stairs. It stopped at the bottom, cocked its ears towards the back of the house, then decided to head in the other direction.

No mistaking it, that was Toby.

Which meant…

"This is Molly's house?" he asked, turning back to Mary.

Mary nodded, hands in the pockets of her jeans.

"But…she's moved," he said, trying to fathom it out. "Why has she moved? Where is this?"

His first thought was that she had moved to another part of the country – after all, the house was bigger, would be more affordable outside of London – and he became immediately aware of a creeping panic setting in.

"Calm down, it's still London," Mary said. "Bit further out, that's all. Nice area. Quite trendy, actually."

It sounded hateful, but he didn't say anything; it couldn't be hateful, not if Molly had chosen it.

"Shall we go in?" Mary asked, standing by a closed door. "Sounds fun in there."

He tried to move, but the soles of his shoes seemed to have sprouted roots that twined into the floorboards. In the end, he felt Mary's hand in the centre of his back, giving him a little encouraging shove.

When he opened the door, the noise escalated, and he didn't know where to focus his eyes first. In the corner, by the bookshelves that were built into a recess, was a six-foot fir tree, the base of which was entirely swamped by a sea of wrapped Christmas presents. Sherlock heard a familiar, trilling laugh, and his gaze swivelled to a brightly-coloured patchwork wingback chair not far from the tree, where Mrs Hudson was sitting, mince pie poised between plate and mouth. Sitting across for her – the supplier of whatever joke had tickled her – was Lestrade, ludicrously perched on a beanbag and holding a glass of something garishly pink and sparkling.

They looked a little older, Sherlock conceded, but how much older? He couldn't tell. Lestrade had been grey (or a bloody 'silver fox', as he laughingly insisted on terming himself) the entire time Sherlock had known him, and Mrs Hudson …well, he'd never dared deduce her age in the first place ("younger than the Queen, and that's all you need to know, dear" – not exactly narrowing it down).

Keeping his back to the wall, Sherlock edged his way into the corner of the room furthest from the tree, giving himself a different view of proceedings. Mary silently followed him, perching on the arm of an unoccupied armchair. On the shelf at his eye level, Sherlock noticed a small, stuffed mouse riding a miniature plastic skateboard.

The room in which he was standing led around a corner into what was presumably a kitchen, but before Sherlock could properly get the lay of the land, he heard John's voice ring out.

"Don't run!" he called, apparently futilely, because a split second later a young girl came tearing around the corner. Sherlock could only take in the head of blonde hair at first, but when she stopped, there was instant recognition – the roundness of an infant had been replaced with more defined facial features, but it couldn't be anyone else.

"My little girl," Mary said, softly, behind him. "Just not quite so little."

Sherlock stared at Rosie, all elbows and knees and gaps in her teeth. She had to be, what, six? Seven? He watched as she came into land beside the sofa, immediately snatching up some kind of plastic sword before dashing off again. Seconds later, Rosie was back, but not alone – this time, she was hanging onto the collar of a very large, clearly very patient, Labrador Retriever. And trailing behind, shrieking with delight, was another, younger child – a boy.

Did John have another child? Sherlock glanced surreptitiously at Mary, hoping to be guided by her reaction, and immediately he saw her face, he knew he was off-track.

Confirmation came when he first heard Molly's voice and then saw her enter the room, slinging the tea-towel she was carrying over her shoulder so she could use both hands to hustle the dog back out of the living room and away from the chaos. Sherlock watched her go to the smaller child, hoisting him on her hip and murmuring something to him as she picked dog hairs (or possibly cat's) from his brightly-striped jumper. The boy's expression was almost a mirror image of hers. With dark hair and eyes, he was about two years old, Sherlock gauged, possibly closer to three; his short legs not yet met around Molly's waist as she held him.

It was only then that Sherlock noticed what should have been blindingly obvious – Molly was pregnant.

For a split second, it felt as though something was choking him from the inside; his breath just refused to come. He couldn't bear to look at either her or at the little boy, but at the same time he couldn't help himself.

"The children…" he said, hoarsely, trying to ignore the gnawing ache in his chest. "Are…are they…"

Mary looked up at him, those big eyes suddenly no longer playful.

"No, Sherlock," she said, gently. "They're not."

He had to balled his hands into fists just to keep himself together, trying to recall every single exercise he'd taught himself for maintaining a cool exterior and keeping his traitorous heartrate under control. It worked in a Serbian prison, it served him well in an interrogator's chair – it even gave him the upper hand with Irene Adler. But it appeared that his unconscious mind had presented him with the one thing that could undermine that training completely, could undo him absolutely. His conscious mind would never allow him to even consider it.

Watching Molly lead the little boy back into the kitchen with her, he now strained to hear those unseen voices, to isolate them from the general chatter and ambient noise. He heard one. Still indistinct, but the low tones of a man's voice, someone other than John.

"Who…who is he?" Sherlock asked, his gaze trained on the floor.

Mary slowly stood, put her hands in her pockets again.

"It doesn't matter," she said softly. "You don't know him."

If that was supposed to comfort him in some way, it didn't, particularly when she could hear that Mary was trying to disguise the sadness in her own voice.

He tried to block everything out for a moment, to draw a screen over the scene in front of him.

"Where am I?" he asked, fearfully. "What…what am I doing right now?"

At this, Mary gave a slight shrug.

"Working," she replied. "It's all you've really done since she got married."

Sherlock felt his hand come up to cover his mouth; all of a sudden he felt another swell of nausea.

"You were invited today," Mary continued. "You're always invited. John's probably given your apologies – you'll add your own by text later on, once everyone's gone home."

He stared at her helplessly for a long moment, feeling himself sink deeper into the depths.

"I can't-"

Sherlock bolted through the door, praying that he would find his living room on the other side of it, that he would finally surface back in the real world.

Instead, he was back in the hallway, with the pictures and the hat and the mittens and the stupid, stupid cat, who was now curled up on a discarded coat at the bottom of the stairs. He turned away until all he could see was blank wall. Silently, Mary joined him, closing the door behind her.

"I…I need to know what happened," he said, turning to her. "Tell me how this happened."

Mary looked at him, scanning his face, perhaps assessing whether she thought he could stand to receive the information. This made sense; he wasn't sure himself that he could. But what was the alternative?

"Okay," she said softly, plainly. She took him by the wrist and gently pulled him to sit down with her on the floor, their backs up against the wall next to the door. Sherlock brought his hands to his knees, felt his thumbs working into the hard bone of his joints.

"Molly asked you," Mary began. "She felt that the two of you had become closer, that perhaps you were on the verge of something – so she told you again that she loved you, but said that she needed to know once and for all where she stood, whether you could see any sort of future with her. Said she couldn't go on wondering, didn't want to live in limbo any longer. It took you by surprise; you weren't prepared for that kind of frankness from her."

Sherlock swallowed thickly, quickly brushing his eyes with the back of his hands.

"What…what did I say?"

"You told her that you couldn't give her the future she wanted," Mary replied. "That you cared about her and valued her friendship a great deal, but it couldn't ever be anything more."

Something deep within Sherlock's chest gave a sharp crack. He wanted to ask why he would say something like that, but it sounded all too like him for it not to be true.

"Molly accepted it," Mary went on, softly. "Took you at your word, respected that decision. She did what she needed to do to come to terms with that – accepted a six-month teaching post up in Liverpool, and took herself away for a while. Long enough for her to grieve for you both, for you to establish separate lives again. You know Molly – she's a romantic, but she's pragmatic too; has that strong sense of self-preservation."

He was openly crying now, the torment was too much; all he was achieving by trying to stem the tide was a wet jacket sleeve.

"So he…is that where he…came from?" he managed.

Mary's gaze flicked to the door they had just left, before returning to him.

"No. She met him in London, when she returned. He's a doctor; not long moved to the hospital."

Sherlock hung his head, his vision blurred and his shoulders shaking. He felt Mary's hand come to rest on his, on top of his knee; felt her lean into his side. She squeezed his hand, which only seemed to wring more tears from him.

Eventually, he looked up at her, the blood from his face gradually draining back to its normal place.

"Please," he began. "Please, Mary, as my friend, I need you to tell me. What I'm seeing here…I need to know whether this is really how it's going to be, whether it's too late to…or whether things could change if I…I act differently?"

A tiny smile played at the corners of her mouth.

"Do you think you might want to act differently?" she asked.

He turned away, feeling a sudden, overwhelming wave of embarrassment and shame, knowing just how transparent he must seem.

"What are you afraid of, Sherlock?" she asked gently, giving his hand a little jiggle. "Tell me. And don't give me any of that rubbish about putting her in danger – Molly's a big girl, she makes her own decisions. She knows what being with you would mean."

Sherlock took a deep breath, stared at their joined hands.

"I know what it's like," Mary continued. "Do you think I wasn't terrified when I realised I'd fallen in love with John? That wasn't part of the plan. I never expected it – certainly wasn't looking for it. It's scary when the path that you think is set starts to crumble away, but…maybe it's a sign that it's not your path any longer."

He took this in, tried to order his thoughts.

"I'm…I'm scared of hurting her," he replied, his voice little more than a whisper. "Being less than she deserves; letting her down. Not knowing how to…do this."

He sensed Mary nodding, taking this in. Then…

"You worried about…?"

She performed a slightly filthy, playground-level mime with her fingers, and gave him a wink. When he looked at her in shock, she eyed him with amusement.

"I mean, that you might be a bit rusty?" she elaborated.

Sherlock wrinkled his nose, feeling the heat rushing to his ears.

"Isn't it one of those things that's like riding a bicycle?" he mumbled, trying to sound dismissive.

Mary gave a snort of laughter.

"And when was the last time you did that, either?"

He managed a small, wan smile.

"Aahhh, you're surprisingly sweet sometimes, aren't you?" Mary grinned, nudging him. "Listen, don't worry about that. Just…let those things happen in their own time. And Molly will keep you right."

Sherlock let out the breath he'd been holding.

"Probably best if you don't mention bikes, though," Mary added, clamping down on a smile. "Molly's not going to be swept off her feet by that comparison."

Sherlock let out a sniff of laughter. The notion of sweeping Molly off her feet was equal parts thrilling and pants-soilingly terrifying, but he felt a small measure of comfort knowing that Mary – even this fictional conjuring of Mary – believed he could do it.

But, he acknowledged, 'could' wasn't necessarily the deciding factor.

"I…I need to know, Mary," he whispered. "Is she happy?"

Mary looked at him, studying his face with an odd look of wonder, as though she was seeing him for the first time. Then she smiled, took a small breath.

"She didn't just 'settle', if that's what you're asking," she replied. "He's a good man. But…you're a good man, too. And you," – she prodded his leg – "William Sherlock Scott Holmes, have the distinct advantage of being the love of Molly Hooper's life."

It was now his turn to look at her in faint wonder. Never before had he thought about himself in those terms – and the concept on its own would have seemed utterly risible to him until a short while ago. If he wasn't careful, his status with Molly could feel like too heavy a weight of responsibility, too much to live up to, but he couldn't – wouldn't – allow it. He needed to justify Molly's faith in him.

Mary got to her feet, holding her hand out to him again. He took it, a strange exhilaration flushing away the aches in his limbs.

"It's time to get on with it," she smiled, as they faced each other. "Go on. Go and be happy, Sherlock; tell Molly how you really feel, make a life with her, and start believing in Sherlock Holmes the man as well as Sherlock Holmes the legend."

She smiled, straightening out his jacket for him.

"And then the two of you need to get started on making some terrifyingly brilliant babies," she added. "Because Rosie needs a few cousins."

There was unabashed mischief in her eyes, and Sherlock laughed, a final, rogue tear tumbling down his cheek. They watched each other for a few seconds, and Sherlock had to remind herself again that this wasn't real; she felt tangible, present, as though she'd just popped out to get milk and now she was back.

"It's been kind of fun, hasn't it?" Mary said, with a smile that betrayed a little wistfulness. "I mean, a bit traumatic, obviously, and I didn't want to make you cry but…it's going to be exciting, Sherlock. An adventure worthy of both of you."

Oh, she wasn't wrong there.

"I should say, though," she continued, "that if you chicken out, I'm going to come back and haunt you for real, and it will be bloody annoying. Experiments ruined, violin permanently untuned; your sock drawer just one big, disorganised mess. I will be one seriously vengeful spirit, Sherlock mark my words."

He laughed again before allowing himself to be drawn into Mary's arms, stooping so that she could rest her chin on his shoulder.

"It's time to go back," she whispered. "It's been a hard watch for both of us."

Sherlock felt the sadness in her voice vibrate against the skin of his neck. She pulled away, holding him at arm's length.

"Look out for John for me," she added. "Keep him busy, say something nice to him occasionally, and don't let him buy terrible clothes."

Sherlock chuckled, dipping his head to press a kiss to her forehead. With his eyes closed, he felt Mary's hand gently pat his cheek one more time.

"Go get her, Sherlock Holmes."