Disclaimer: I don't own any of the iterations of Sherlock Holmes that exist in the world, but in particular no the BBC's Sherlock, nor do I own Buffy the Vampire Slayer.
It was never easy to be told you were wrong, and Buffy had to force herself to go straight to Sherlock and John's flat. When she paused outside his door, steeling herself, he called from inside, "Just come in, Anne."
She didn't give him any chance to rattle her. "I'm sorry," she told him. "I should never have called your vocation a job, and I shouldn't have taken it the way I did. I just sometimes forget you're not like other people, and I really shouldn't."
Sherlock sat up from where he'd been lying on the couch. "John explained your prior experience with others belittling your Work," he said in tacit acceptance. "Having seen your interaction with your sister, I can understand the assumption. One cannot avoid placing people into categories for ease of mental access."
"No," she admitted. "But you're different. In a lot of ways you're like Spike. He wouldn't have understood the strain of being bored all the time with too much junk coming in, but he did understand being a hunter. You don't really get, in your gut, the whole hunter-killer thing-"
"On a visceral level," Sherlock offered.
"I can't ever use that," Buffy told him. "No one would ever believe it. I have a reputation, you know." She smiled. "But thank you. Maybe sometime when I'm with people I don't need to keep up the rep with I can try talking all smooth and sophisticated." Then she sighed. "The point-"
He interrupted again. "You are unused to having your deductive capabilities respected nor your genuine reverence and appreciation for your work respected."
Someone else might be annoyed with the interruption, but the fact that he was just saving her from having to put into words something that she didn't always fully see in herself made Buffy smile. "John said you were having a bored day and Greg wasn't sending you anything either. I know it's not totally your thing, but a few of us were having to track down what a new bad guy in town is up to. You want to come along? We're going to be doing a lot of book research, but I need to go out and get a few clues to narrow the search field down."
He rolled to his feet with a smile. "I would be delighted to offer my assistance."
And he was. It was great to go out walking the places people had seen the Fyarl demons running around, clearly working on someone else's orders and have Sherlock's sharp eyes pick out details she didn't notice. He was taken aback somewhat at her beating some answers out of one once they'd caught it out, but unlike Riley he rolled with what she was doing and how she was doing it. Between them they were able to collect enough information to narrow the book research parameters to tell her what sort of demon they were looking for and that it did need to be killed with a specific magical weapon and did it so much more quickly than she could have done on her own.
When they were about to leave he received a text from Greg about a new case and left her to do her thing without pulling a Xander or Riley about how she needed protection or help when she hadn't asked for any. Buffy just kissed him goodbye and asked him to text her when he was through, and that she'd drop him a line when she was done.
A couple weeks later, Sherlock had told her he was having dinner with John, and she'd been forced to go straight from a meeting with Mycroft to a demon who owed her a favour. Then she raced over to the restaurant John and Francesca were having their one-year anniversary date at, getting there at the same time Sherlock waltzed up to their table. John was looking aggravated and amused, Francesca was just looking like she was going to curse Sherlock with boils.
Buffy strode over, sliding into a chair next to a surprised Sherlock, and said with her best society girl smile, "Happy one-year anniversary, guys." She slid over the small envelope with its little bow. "Here's to more happy years."
Francesca opened it and she flicked through it. "These are . . . VIP vouchers to see Tottenham!"
"Yup," Buffy said. "Stjepan said there were also some vouchers or something for some other freebies to go with." She leaned into Sherlock, who eyed her suspiciously, adding, "We hope you enjoy."
John giggled. "Thank you Buffy. And Sherlock," he smiled a little ruefully at his friend. "I'm sorry I haven't been available for the last couple cases," he said. "I missed you too."
Sherlock blinked as it all went through his internal filing system. Suddenly he looked apologetically at John, but made no move to leave. Francesca looked resigned and Buffy set herself to her well-practised skills as a social butterfly. Between John and Sherlock's inevitable lockstep, which kept John from being too angry with the man who was more important to him than anyone else, and John's clear and determined affection for his girlfriend, it wasn't too hard at all for Buffy to get conversation moving.
In a pause, Sherlock finally asked her, "How is it that you have manipulated the situation and yet avoided appearing utterly obnoxious?"
She eyed him. "Unlike your brother, I'm not making it totally obvious to everyone that, not only am I manipulating them, but the fact that they can't get out of it makes them inferior. Instead, I've made it obvious to John and Francesca that I'm trying to save their romantic dinner that you crashed, and make it less horrible by allowing us all the polite fiction that this is a double-date in celebration. I'm making it clear that my intentions are to make people happier-"
"While Mycroft cannot help but use the opportunity to demonstrate that he has power and is exercising it," Sherlock said. "No, I see."
When Francesca and John started getting distracted and moony-eyed at each other, Buffy was able to get Sherlock's attention and not break up the romantic moment on the other side of the table. When they were about to go their separate ways, Buffy promised Francesca she would do her best to defray Sherlock's interruptions when she could. Sherlock and John didn't notice because Sherlock was telling John all about a case he was hoping Greg would call him in on.
Buffy's slaying went normally over the next few weeks, the usual smattering of vampires killed before they could kill again, a demon or two with the usual unusual requirements for murder and mayhem and one or two types on the evil side of things with ambitions. Sherlock had his cases, and in between they dated. It was different from any relationship she'd ever had before, because one-night-stands and pre-slayer boyfriends aside, all her actual relationships had been with other people who were hunting demons alongside her. For the first time, she and her significant other had, for want of a better term, different jobs. At the end of the day, when they met up she could tell him about her day and he could tell her about his and they hadn't been in each other's pockets all the time. The space was refreshing, and Sherlock's unusual nature and his calling meant that he never resented their interruptions.
She had had to break a date with him in order to deal with the usual demon hijinks, and sent him a text the next morning, knowing that he wouldn't see the point in being told she was done if she was just going to be asleep and thus unavailable, then checked her email. Giles, through Willow, was asking her to see Mycroft Holmes. It appeared that some of their funding had been choked off on his orders, and she was the person to find out why and get it reinstated.
Arriving at the office where Sherlock's older brother normally held court when he was being conventional about his job, Buffy found herself eyeing her surroundings. Something was off in the office. None of the usual people were there and everywhere were the small signs of brand new hires. It felt wrong, and Buffy hastily shot Sherlock a text about it, asking whether he'd heard something about a shake-up in his brother's department. "Please, come in Ms Summers," came Mycroft Holmes' smooth, condescending and irritating voice.
Gathering herself to play the game, Buffy walked into what seemed to be the lion's den. Something was wrong, but without any sort of clue, and Mycroft Holmes was as smart as his brother, which meant there were no clues, all she could do was brazen it out and hope Sherlock was able to help her work it all out. "So, rather than playing too many word games," Buffy said, "Please tell me why you've cut the funding for my organisation."
"Ms Summers," he said with the sort of condescension that would have made her punch him had she been only a little less mature. "I do not know what is being done with that money-"
"Nor are you going to," Buffy told him. "I do understand your complete inability to trust me and mine, but the contract is quite binding on the government of the United Kingdom-"
She was cut off, but not by Mycroft Holmes. The voice of a man she had thought dead, spoke. "But that contract was with the Council of Watchers, not your little ragtag band of miscreants, Miss Summers."
The apparition that had spoken looked like a zombie. His skin was grey, his hair, which hadn't been all that thick to begin with was wispy and patchy, and he looked like he'd been through a fire. The ridiculously immaculate suit and tie with the coat he'd worn overtop it every time she'd seen him looked particularly out of place. "How did you survive the explosion? There wasn't a trace of you. Giles made sure to look for anyone who might have made it."
Quentin Travers glared. "And once we were out of the way you took the funds and started wasting it on fripperies."
"You mean, like paying the slayers for doing a job?" Buffy asked.
The former head of the old Watcher's Council sneered. "Do not try to pretend you earned that money. That is to be spent on-"
"Retreats to the Cotswolds?" Buffy asked. "Printing out books that tell me it's more important to learn the proper names of martial arts moves in Japanese than how to survive the Apocalypse? Paying off the people running mental wards for the clearly criminally insane so you can get them turned into vampires then throwing an unprepared teenaged girl to her death?"
Sherlock's brother looked shocked. "I do not know what ridiculous vendetta you have against each other, but you will remove yourselves from my office."
"Sit down, Mycroft, there's a good boy," Travers said, as he pulled an amulet out of his pocket. A glow lit the amulet and Buffy saw the other man was shocked as his legs seemed to give out from under him, dropping him onto his chair.
Buffy eyed him. "How long, Travers? How long have you been using that on him?" She paused a moment, thought back to the hints Sherlock had said about his brother, then hissed in realisation. "It's been seven years, hasn't it?"
Travers blinked, taken aback. "How could you possibly know that, you ignorant, American bint?"
"Why does it matter?" she asked, not willing to give him an inch. "But I'll tell you this, you're going to stop, now."
The bastard smirked as she launched forward, security forces bursting through the doors, called by Mycroft Holmes, guns ablaze. Instinct and Slayer superpowers let her change direction to anticipate some of it, but Buffy couldn't dodge every tazer and every bullet. Pain washed over her, and she went down, unsure of whether she'd been shot, tazered or something else.
Sherlock didn't receive Anne's second text until a hour after it was sent. The terse message, Entirely new staff at Mycroft's office. Shake-up? was very concerning. Several texts later he was certain she was not responding as a result of some sort of difficulty or danger. He arrived at the government office to see that Mycroft's favourite flunky was missing. He elbowed his way into his brother's office, seeing a misplaced pen on the floor in a corner where it never would have rolled on his brother's watch. One of the paintings was off-centre and the floor had been hoovered within the last twenty minutes.
Anne was also correct, in that not one of the people there was a familiar face. "Where is Mr. Holmes?" Sherlock demanded.
A woman sitting at the secretary's desk gave him a coy, smug smile, and said, "Mr. Holmes is not available. Can I take a message for you, sir?"
Sherlock looked her up and down. At a casual glance she seemed like a normally attractive woman, but her head was tilted at a slightly odd angle. Not enough that it would be noticed, but the human neck didn't quite bend that way. Someone not as finely attuned to the capabilities of the human body wouldn't notice it. There was the smallest of blurs to be seen around a necklace she was wearing, and her teeth didn't seem to quite sit correctly in her mouth. A sudden suspicion made him straighten up and glance around the room.
Research into Anne's collection of books, chats with demons in London's supernatural underworld and his usual observational abilities had honed Sherlock's grasp of the nonhuman masquerading as human. Not one of the people in that office was human. "How about you take a message to whomever has purchased the services of the demons in this office and inform that person that I would like to speak about what has been done with the Slayer and my brother." He smiled in that way Donovan had repeatedly informed him made him look 'disturbing'.
The demon woman blinked, then said chirpily, "I'm afraid, sir, I don't know what you're talking about."
It was clearly a signal, as the others in the room converged on him. Sherlock, knowing he had no other options, dashed past them in haste and barely made it to the entrance hall of the building and the public area still staffed by people who had known (and disliked) Sherlock for the years his brother had worked there. He deliberately sauntered past them, throwing a triumphant smirk at his pursuers, who couldn't afford to make a scene. Once outside he caught a cab and went straight to see John.
"Sherlock?" John was dishevelled and had clearly been in the midst of an amorous encounter with Francesca.
"There appears to have been an altercation at Mycroft's office. Anne had informed me that the denizens in his office had all been changed, but when I arrived there was evidence of concealment of a violent encounter and neither Anne nor Mycroft are in evidence. Moreover, it would appear that all Mycroft's people have been replaced with what appeared to be demons."
John frowned. "Francesca, should we try to scry for them?"
She briskly stood. "I'll set up some wards," she glanced at Sherlock, "Specifically wards meant to prevent other people from noticing the scrying or backtracing it to here."
John promptly began to collect some objects and told Sherlock, "Do you have anything of Buffy's for me to use as a focus?"
"I shall fetch something at once," Sherlock said, leaving for Anne's flat. Along the way he set his Homeless Network to search for anyone who might have seen Anne or Mycroft taken. At Anne's flat he hastily searched out a few objects of hers that he thought, based on her particular tendencies to sentiment, might resonate in a way John would find useful.
When he returned there was a large mirror centred in the room in a way conducive to easy viewing. John took the photo, stake and diary, handling each briefly, before settling on the stake. He then closed his eyes, recited a cantrip and Sherlock watched, anxious, as the mirror glowed a moment, then went blank again. John frowned and said, "I can't find her, and it feels like someone's blocking me. Where's Mycroft's office at?"
"Whitehall," Sherlock replied, then clarified with the precise address. He gripped Francesca's arm a moment when she seemed about to speak and disrupt John's concentration. Clearly John was planning to attempt backtracking along Anne's route in some way.
John's face was a mask of concentration, but as Sherlock watched, the mirror brought the exterior of the office into view, moved around to the shipping entrance in the back, slowly moving along into the car park, then to the street, then started north. For a moment the image followed, then it stopped dead at an intersection. The image vanished and John sagged. "They must have put whatever magical safeguards they used in place when they reached that point."
"Thank you, John," Sherlock said, and began two trains of thought, calculating possible directions and destinations based on the initial directions of travel, attempting to determine a reason for the kidnapping of both his brother and Anne, texted one of his number of homeless informants with mobile phones, and then began to hack into the CCTV system to see whether he could gain access to security footage for that place and time.