So originally this was a one shot but it got a little out of hand and its four chapters instead. I also picked the worst year to write a Halloween fic as I'm on holiday the week before Halloween with no wifi which is why you are getting this in bits; two chapters before I go away and two when I get back on Halloween and the day after.

Normal disclaimers apply.

Chapter 1

It was absolutely, most definitely Autumn and Molly Hooper was happy. This had always been her favourite time of year; the time when she could put away her summer dresses and throw away the sun cream, which she had to constantly cover herself in in order to not risk sun burn. Summer was always too hot and sticky and Winter too cold and slippy. Spring was OK but still held memories of her dad's death but Autumn...Autumn was the time she could get out her favourite jumpers and cardigans. She could wrap up warm and see her breath puffing out before her in the cold air. She loved seeing the leaves turn golden and red and brown; kicking her way through piles of leaves in the London parks on her way home from work, with her long scarf wrapped cosily around her neck.

It was also the time of Halloween and Bonfire Night and this year promised to be extra fun as she and Mary had bought tickets to a big fancy dress Halloween party being held in a posh hotel in the centre of town. They had seen it advertised a month or so ago and hadn't been able to resist. It wasn't often that Molly splashed the cash, she was normally quite safe and conservative but Mary was a bad influence and before she knew it they not only had tickets but they had booked a hotel room each as well.

Her only two problems were what she should go dressed as and who on earth she should go with. A week or so after buying the tickets Molly had gone round to see John, Mary and the baby and had broached the subject.

'Well, what about Sherlock? He likes dressing up, isn't he supposed to be really good at doing that when he's under cover and spying on people?' Asked Mary just before John choked on his coffee.

It took him almost a minute of coughing before he could recover and even then there was still the occasional throat clearing taking place, 'hmm, no. I don't really think it's Sherlock's thing. I can barely even get him to come to the pub for a pint, let alone dancing at a party. Let's not forget how he left our reception early. No, it's really not something he'd be interested in.'

Molly blushed lightly at the idea of going with Sherlock but agreed with John, 'anyway I think my days of asking Sherlock out are behind me. He only ever changed the subject in the past or blatantly ignored me so no, maybe I'll just go alone.'

'Well you could, we're more than happy to have you come with us but it seems a shame to waste the extra ticket. There must be someone else we can think of. Who else is single?'

There was silence in the room as John read his paper and Molly and Mary tried to come up with someone. Mary huffed and shoved John's knee, 'come on love, help us out. You must know some single guys.'

'Nope I really don't...although?'

'What? Who, you've thought of someone haven't you?' Mary grinned, 'who is it?'

'Well, I mean you probably wouldn't want him as a date but he is single.'

'Oh for God's sake John who is it?'

SHSHSHSHSHSHSHSH

'Lestrade, Gavin Lestrade. That was who you came up with.' Sherlock snorted in disgust and sat back in his chair, the microscope set up on his kitchen table forgotten momentarily.

'It's Greg and yes, what's wrong with him?'

'What's right with him? I can't believe you've foisted a sexually frustrated, two time divorcee on my pathologist.'

'She's not your pathologist and there's nothing wrong with Greg. He's been a bit down since he and his wife divorced again last year and...and just because they're going together doesn't mean they're on a date. It just means they're two friends enjoying a night out together. Anyway what makes you think he's sexually frustrated?'

'Well, for one thing he's unlikely to have slept with his wife in a while given her predilection for young gym teachers and secondly it's obvious...you must have seen... from his ties.' He rolled his eyes when John just looked at him blank faced but didn't explain. 'If you think he isn't going to make a move on Molly then you're more of an idiot than I had already pegged you for.'

'And so what if he does, they'd probably make quite a good couple. It's about time Molly met someone else, it's nearly been a year since she broke up with Tom and it's not like she's had any other offers is it?' John said pointedly.

Sherlock just thinned his lips and frowned before turning his attention back to his slides.

'And anyway how come I didn't know about this night out until now, two days before the event, I don't remember receiving any sort of invite.'

John just rolled his eyes and picked up his laptop, turning it on so he could check their emails for any possible cases. Sherlock was always more grumpy and highly strung when there was nothing going on. 'Yes, because you would have been so interested in an evening of dancing in fancy dress. Right, how about this one, chap here says someone is moving the bench in his front garden every night, thinks it's ghosts or government officials spying on him?'

'Boring, he's paranoid and his neighbours are doing it for a laugh...so what are you going as?'

John looked up from the laptop, 'sorry what?'

Sherlock looked at him in frustration, 'keep up John, it's not hard, what are you and Mary going dressed as? You said it was fancy dress.'

'Oh right, well it's not important. What about this one, a lady in Birmingham wants us to investigate the disappearance..'

'Nope, boring.' He stood up and made his way into the front room, 'now why aren't you telling me? Obviously something embarrassing which means Mary has chosen it. You're married to an ex-CIA assassin what on earth would she choose for a Halloween fancy dress...?' He tilted his head to one side as he stared at John thinking through the options, 'James Bond and Miss Moneypenny but she's Bond and you're the PA?'

'Yes, very funny Sherlock. If you must know we're going as Gomez and Morticia from the Addams family. And, I'm suspecting you've never even heard of them.'

Sherlock laughed, 'aren't you a bit more cousin IT height-wise than Gomez?'

'Seriously the Addam's family. You delete the solar system and keep the Addam's family.'

Sherlock shrugged, 'some childhood memories are worth keeping. What about Molly and Gavin, what are they going as?'

'Why on earth are you so interested in this ball? Do you want to come with us or something?'

Sherlock scoffed and turned back towards the kitchen, 'no, of course not. I'm just filling a gap in my day until you get round to finding me a case. Which, I might add, seems to be taking you an inordinate amount of time. There must be something out there.'

John turned back to the laptop and carried on skimming through the emails, deleting the spam and obvious trolls. 'Dr Frankenstein and her monster.'

Sherlock stopped in his tracks and looked back round. John looked at him and clarified, 'Greg and Molly...he's the monster obviously, she's apparently going to be a sexy scientist.'

SHSHSHSHSHSHSHSH

It was only later, after John had left, that Sherlock allowed himself to think about their earlier conversation and his strange reaction to it.

At first he'd just felt a sense of irritation at hearing about everyone's plans for the weekend. He couldn't help but feel a little hurt and left out that he hadn't been invited at all. He knew they didn't know it but he did love to dance and dress up. Why would they not have even asked him? He felt further incensed when John mentioned that Lestrade was dating Molly. Yes, he knew that John had said it was platonic but who was he kidding; the outfits, the drinking it was all going to lead to one eventually outcome and that was Gavin Lestrade having sex with his Molly. It made his teeth itch just thinking about it.

That just led him on to thinking about the strangest reaction of all, the one he'd had when John had mentioned Molly dressing up as a sexy scientist. It had stopped him in his tracks and had taken all his skill at dissembling to not reveal his inner turmoil to John.

The images that those two words had put in his head had shocked him to the core. It was as though John had thrown a bucket of cold water over him. His pulse had thundered in his ears and he had found it difficult to swallow, all the moisture seemed to have evaporated from his mouth.

As he lay on his couch three hours later he explored his reactions in the safety of his mind palace.

He thought of Lestrade touching her and could feel his hands wanting to fist, his insides twisting uncomfortably, why did he not like the thought of the two of them together? He imagined the DCI with Donovan or another female and found it didn't bother him a bit so it was nothing to do with the police officer and everything to do with Molly.

What did it all mean? Molly had gone on dates before and it had never bothered him...had it? He remembered his dismissal of Jim, his satisfaction when her engagement to Tom had fallen through, his advice to her to stop dating. It was much easier for him when Molly was single, it just meant that she was more available to help him with his cases. It was just related to the work, the work was what was important. Nothing to do with Molly herself.

But then what about his reaction to her costume idea...sexy scientist. He let the words roll around in his head but it all came down to one disturbing image in his mind, Molly Hooper in heels and a lab coat, her hand on her hip beckoning to him with her finger with nails painted a blood red. He groaned and opened his eyes in shock. As he made to sit himself up he realised he had a partial erection. God no! He ruffled his hair in frustration. Really, feelings, that's what this was all about?

He lay back down, he needed to delete and delete fast before they spread and infected his whole being.

It didn't take him long though to realise he was too late. Somehow over the past six years Molly Hooper had wormed her way into the heart he had never admitted to having and all of a sudden the thought of letting her hook up with DCI Lestrade had him reaching for the phone and booking his own tickets to the ball. Now all he had to do was come up within outfit and a partner, after all he was Sherlock Holmes and he wasn't turning up unaccompanied; the former proving easier than the latter.

MHMHMHMHMHMH

It didn't take long for Saturday night to come round and Molly excitedly met up with John and Mary before they all made their way to the hotel. As they had rooms booked they were changing at the venue and meeting Greg in reception just before the event kicked off. They'd left Mrs Hudson at John and Mary's already excited about her own night out at theirs looking after the baby, 'oh it's been years since I had a chance to babysit. Now don't you worry everything is going to be fine. I have your mobile number if I need to get hold of you. Just go and have a fantastic night and I'll see you in the morning, you can tell me all about it.'

John responded, smiling widely at her 'thanks again Mrs Hudson, you've no idea how much Mary and I are looking forward to a night out. Was Sherlock alright when you left him. I tried texting him earlier but he hasn't replied.'

'Well, it's the strangest thing, I think he might have a date tonight. He said he was going out and just before I left that girl came round, you know John that one he was seeing before.'

John couldn't help but notice Molly's face falling. He frowned as he looked at Mrs Hudson, 'you mean Janine? Mary's bridesmaid?'

'Yes, that's the one. She had a bag with her and everything, looked like she was planning to stay the night. Anyhow, go and have fun, I think I can hear the baby grizzling so I'd best get on.'

Molly plastered a fake smile on her face as the three of them climbed into the waiting cab, 'well that's nice...for Sherlock I mean. I didn't know he was dating anyone at the moment.'

John was still frowning, 'no neither did I. What the bloody hell is he up to?'

'Well, maybe he genuinely likes her and they've set up a date night.' Molly felt a little sick as she said the words. The only thing worse than Sherlock not being interested in her was the thought of Sherlock being interested in someone else.

Mary shook her head agreeing with her husband, 'no, he's up to something. Oh well, you can go round tomorrow John, and find out then. Let's forget it for now, we don't want Sherlock spoiling our night now do we.'

Once at the hotel Mary divided her time between her own room and Molly's. She and Molly had ordered up a bottle of cheap champagne to get them started and were giggling as they put on their outfits and make up but Mary had to keep checking on John to make sure he was dressed how she wanted him to be. By the time they went downstairs Molly had all but forgotten her upset over Sherlock. She had laughed when she'd had her first sighting of John. His hair had been slicked back with what looked suspiciously like black boot polish and Mary had stuck a black moustache on his top lip. He was dressed in a garish 1950's style striped suit.

Mary meanwhile was in a figure hugging floor-length black dress with strips of black netting sewn to the bottom of it and she had on a long black wig and bright red lips. They both looked amazing together and Molly had already taken photos on her iPhone to upload to Facebook later on.

Her own costume felt a little more revealing than she'd expected and she was feeling very self-conscious. All she had on was one of her old lab coats that Mary had helped her adjust for the night. Against Molly's better judgement Mary seemed to have taken about ten inches off the bottom as well as taking in the waist. When she'd put it on in her hotel room she had nearly died at how short and tight it was but Mary had just tutted and told her she looked amazing.

She'd teamed it up with black stockings, the tops of which you could almost see, she was really going to have to be careful not to bend over, and simple but high black stilettos. To finish it off she had a stethoscope and one of the security guards at work had printed her a badge with the title Dr Victoria Frankenstein.

In the end once Mary had finished her hair and make-up she had to admit she looked very pretty and very unlike her normal self. She self-consciously tugged the lab coat down until Mary slapped at her hands, 'stop it Molly, you look hot. You are going to knock Greg's socks off.'

When John finally saw the two of them he let out a long, low wolf whistle before kissing Mary and whispering something in her ear which had her giggling and slapping his chest. He then turned to Molly, 'wow, Molly you look great. You should wear that lab coat to work more often.'

Molly blushed but laughed and thanked him as they made their way downstairs to meet up with Greg.

The next chapter will be posted Saturday, the day I go away. Let me know what you think so far.