I guess this fic could almost serve as a prequel to a previous fic of mine, 'Back-up Plans'. It's almost an origin story for Mary and Molly's friendship, which obviously developed off-screen to the point where Molly was trusted enough to be a godparent.

Just a one-shot set during 'His Last Vow' – hope you enjoy :-)

John was bundling Sherlock back out through the door of the lab into the hallway, his hollow-eyed weirdo dealer-slash-chemist-slash-protégé sloping behind them. Mary could hear Sherlock's self-righteous protests disappearing down the corridor, matched by John's gruff and increasingly pissed-off rejoinders – she should probably catch up with them in case Sherlock tried to make a run for it. Not that he would get very far – he was so bloody high he would probably high-tail it straight into a cleaners' cupboard.

Mary herded Isaac towards the doorway, too. The kid was clearly in no hurry to return home, and to be honest, Mary wasn't looking forward to the conversation with Kate either. Exchanging pleasantries on the front steps or putting the bins out for either other were one thing – extracting and returning the neighbour's AWOL son from a drug den was something that she felt neither of them knew the script for. Bringing Isaac along to the hospital was the convenient option, but Mary also wanted to give Kate some time to cool off once she'd heard that Isaac was alive and basically okay – and the teenager, who was not stupid by any means, could bloody well do with reflecting on his life choices, too.

And the scene that had just played out probably gave him pause. Mary had been as surprised as anyone in that room at Sherlock's very public dressing down at the hands of Molly Hooper – although, she wondered, perhaps the only person who hadn't been surprised was Sherlock himself. He almost looked as though he expected it, and certainly didn't say or do anything to stop her. The retort he eventually spat back at her was completely repugnant and personal, to the point where Mary now felt a pang of guilt that she hadn't intervened (and a slight stab of irritation that John hadn't felt the need to, either).

She glanced back at the door to the lab.

"Listen, Isaac, love," she began. "Are you okay to catch up with John and the others? I'll be along in a couple of minutes."

"Yeah. Alright," he replied, a note of uncertainty in his voice.

"Okay, great," Mary replied. "Tell John I'll meet him at the car. And no trying to give us the slip, right? I'm too tired to go chasing around London after you, and John's got enough on his plate with our idiot friend."

He smiled slightly, wearily, and began trudging down the corridor.

Once he was on his way, Mary quietly back-tracked to the path lab. Through the window in the door she could see Molly; she was sitting at the bench among a pile of paperwork, but Mary could see immediately that her friend's concentration was elsewhere. Well, she thought of Molly as a friend, but now Mary was starting to wonder whether Molly would see it that way - they had barely seen each other since the wedding, and Mary felt utterly shit that she learnt about Molly's broken engagement in the way she had.

She saw Molly rest her forehead in her hands for a moment before taking a deep breath and picking up her pen again. Mary gave her a second before knocking softly on the door.

"Sorry, just me," Mary said, leaning around the door, into the room.

Molly's head snapped up, and she looked flustered for a second, as though she'd been roughly pulled away from her thoughts.

"Oh, hi, sorry," Molly replied, getting up from her stool. "I…sorry, I thought you'd gone with the others."

"I had. But they can look after themselves for a few minutes," Mary replied. "Well, probably not – but hopefully they can't get in too much trouble between here and the car park."

Molly nodded, offering a brief, slightly awkward smile. It was very possible that she wanted to be left alone right now, but Mary couldn't leave so many things unsaid. She did that enough in her life already.

She took a breath.

"Look…I know he's a total bloody mess at the moment, but we'll sort him out," Mary said, hoping that her tone was more convincing than her convictions. "John and I can take it in turns to watch him, we'll talk to him, make him see a specialist if we can – maybe even convince him to go into rehab. I'm sure his brother's got some connections."

Again, Molly nodded – Mary could see she wasn't convinced. She was forgetting, Mary realised, that she was relatively late to the party; Molly had known Sherlock Holmes a lot longer than she had.

"I…I shouldn't have done that," Molly said. "It was totally wrong. He has a problem – a serious problem – and I should know better. I mean, I'm a doctor – 'do no harm' and all that."

"To be fair, Molly, most of the patients you see are usually beyond the point of help, so it's understandable."

She hoped that an attempt at humour might lighten the situation. In response, Molly uttered a short, wry laugh.

"And you weren't reacting as a doctor," Mary continued, gently. "You're his friend, and you care about him a lot, and you're bloody frustrated with him – you were expressing how we're all feeling."

"Yeah, but since when did trying to slap some sense into someone actually work?"

Mary tilted her head to one side, one eyebrow raised.

"Oh, I don't know," she said. "This is you and Sherlock we're talking about. Might have more effect than you think."

Molly looked at her quizzically. Oh, she really didn't know. God, Sherlock was good at hiding it.

"Anyway," Mary continued quickly. "Sherlock Holmes needs a bloody good slap from time to time. It was long overdue."

"I just…" Molly began, looking up at the ceiling for a moment. "It's just so difficult to stand by and watch him do this to himself – he…he doesn't seem to understand what he has, or maybe it's that he doesn't care. I just…I wish he did care, and that he would, I don't know, tell someone if that's how he's feeling."

Mary nodded. She didn't blame Molly for one second – the woman had helped Sherlock fake his own death and keep it a secret for two bloody years, so it must be galling to then see him putting his life in jeopardy, apparently so casually.

"He's claiming it's for a case," Mary said, shaking her head. "But I'm starting to think that maybe he was just bored."

Or lonely, she added to herself. Since the wedding and honeymoon, contact with Sherlock had been sporadic for a number of reasons, and while Mary didn't believe he bore her any ill will – thought she was taking John away from him – three was always a difficult number. Sometimes she sensed John thought so, too.

"I'm sorry he was such a wanker to you, too," Mary continued, taking a step closer to the bench. "That was…that was really shit and underhand. He cares about you, Molly, he really does, but he's not himself, and you know what he's like when he thinks he's cornered. He reacted that way because deep down – not even very deep down, actually – he knows you're right."

She took another breath.

"And I'm going to stop making excuses for him right now," she said. "And for myself. Molly, I'm really sorry that things didn't work out for you and Tom, and I'm so sorry that I'm only finding out about it now. Like this."

She saw Molly's eyes close for a moment; she pushed a stray strand of hair behind her ear.

"It's fine," she said, her mouth a thin straight line as she nodded through a breath. "It just…it wasn't going to work."

"When…?" Mary ventured.

Molly's brow furrowed.

"Um…a few weeks ago. Seems like ages ago now."

A few weeks ago meant not long after the wedding. Even Mary – distracted by the celebrations as she was - had seen the way that Molly looked at the best man that day.

"It was, you know, amicable – sort of," Molly continued, as though she had to explain herself. "It wasn't Tom's fault at all – he's...he's lovely, really. Just…it really was a 'it's not you, it's me' kind of thing. On my side, I mean. It wouldn't have been fair to him to, you know…go ahead with it."

Mary felt a small ache in her chest just listening to her friend, watching her pained expression. She didn't need to ask the question to know that Sherlock Holmes played a significant supporting role in the break-up of Molly's engagement, however indirect.

"I'm so sorry, Molly," she said again, hating how it came out sounding like a tired platitude. "I wish…I wish you'd felt like you could have told me sooner."

Molly looked slightly surprised at this, as if to say really? And she would have had a point – they weren't close, not really. She had only got to know Molly through John and Sherlock, and on almost all of the occasions Mary had been in the same room as her, Tom had been there, too, or else they were all entrenched in case, so there had been very few real opportunities when they could talk. The only other occasion had been Mary's hen do, when Molly had been one face in a sea of women Mary didn't know much beyond the surface; she'd been fun that night, up for a laugh, but possibly not completely comfortable in a crowd.

For Mary, groups of casual acquaintances were safe, and to be honest, she hadn't been looking for a real friend – given her circumstances, friendships were too risky on both sides, and she could never really give a friendship everything it needed to grow and hold firm. But what she'd seen in the lab that very morning somehow changed things – she felt as though she had really seen Molly Hooper for the first time, and for the first time in a long time, Mary knew she had found someone worth forging a friendship with.

"It's fine, honestly," Molly replied, with a sad, dismissive little wave. "I've been working a lot, and you're busy, you've just got married and – god, I'm sorry, I haven't even asked you how you are, how you're feeling?"

Mary smiled. Bless Molly Hooper. Probably having one of the most shit mornings in recent memory, and still trying to remember her manners and put someone else first.

"I'm fine," Mary smiled. "Thank you. I mean, I vomited in the neighbour's hedge as we were leaving this morning, but at least it was fairly discreet this time. I narrowly missed someone's shoes at the bus stop yesterday. You find you lower your standards quite a lot."

Molly grinned in response, almost despite herself, and Mary smiled in return, feeling a measure of relief at the same time. She hated to think of this brave, capable, fiercely intelligent woman quietly grieving a break-up on her own, and quite possibly struggling with conflicting feelings for another man, too, with nobody to confide in. John's assessment of Molly's feelings for Sherlock as a 'crush' was woefully wide of the mark – perhaps it had been that way at one point, Mary conceded, but what she saw now ran much deeper than that. Far deeper than Sherlock Holmes deserved at this point, that was for sure.

"It's lovely, though," Molly replied. "You and John having a baby, I mean – not the, uh, the vomit stuff. You'll be amazing parents, both of you."

Mary found herself feeling oddly touched that someone would think that of them, of her. Happy as she was, she wasn't always certain she was cut out for motherhood. But here she was.

"Yeah, well, thanks to Sherlock today, I think I'm getting a preview of 'tear-away teenager'," she said. "He's usually good for toddler practice."

She saw Molly smile, but could tell it was forced, that the sadness wasn't far from the surface.

At that moment, Mary's phone rung.

"Mary, what's going on?" said John, in a harassed tone. "Are you coming?"

She glanced at Molly, who had turned her attention back to the papers on the benchtop.

"Yeah, John, keep your pants on," Mary replied. "Just coming."

"Okay, but at this precise moment me and Isaac are actually sitting on Sherlock to prevent him doing a runner. And that Wiggins bloke is currently circling the ticket machines looking for loose change."

"Isaac and I, John!" Sherlock the grammar-pendant piped up, presumably from somewhere underneath John.

"Fuck's sake, shut up!" came John's response.

The call ended, and Mary looked to Molly apologetically.

"Sorry, I really do have to go this time," she said. "John's on the brink of serving me with divorce papers."

"Do…do you need any help?" Molly asked, but Mary could see that she wouldn't be up to it, wouldn't be able to face Round 2 with Sherlock Holmes quite yet. Which was probably a good thing, because if Sherlock aimed his guilt-induced venom at Molly again, Mary would likely have to hit him herself – although this time it would be more of a right-hook than an open-handed slap.

"We'll handle it, don't worry," Mary replied, hoping it sounded convincing.

Molly nodded.

"Okay…but, look…I'd like to help if I can," she ventured. "I mean, with whatever you end up doing to help Sherlock. Will you let me know?"

Mary looked at her, amazed by Molly Hooper's capacity for compassion, despite the object of that compassion being an infuriating, self-centred tosser. She found herself nodding.

"Yeah. And listen, Molls, when all of this calms down a bit, let's have coffee, eh? Or lunch?" she said. "There's only so much testosterone I can stand."

Mary heard Molly sniff and saw her discreetly swipe at the corner of her eye before meeting her gaze with a tight smile and a quick nod. She hoped Molly didn't think this was a hollow promise, something said cheaply in the moment as a salve, because the more she thought about it, the more she accepted that a life sandwiched in between John Watson and Sherlock Holmes – exhilarating though it was - could become relentless. She needed a port in the storm. And she needed to bring Molly Hooper into the fold where she deserved to be, if that's what she wanted. John needed to recognise her worth, and at that moment, Mary was fully prepared to plonk Molly in Sherlock's path in as many ways and situations as it took for the man to realise what it was that he was willfully shunning.

But for now, she had a car full of junkies – and a very aerated husband – to get back to.

"I'll text you," she smiled, closing the lab door behind her.