A/N: So this is set to take place where my other Sherlock fic, "A Casual Affair" (which is still in progress at this time), leaves off. After listening to 'Let Her Go' by Passenger I became rather inspired and this would just not leave me alone until I had it written. If you don't want any spoilers about what is to come in "A Casual Affair", I guess I would avoid reading this. There are broad plot points revealed but everything will be further explored so I don't think it would ruin anything. Hope you all enjoy! If you've got a moment, please leave me a review and let me know what you think! :)

And thank you to the lovely reader who alerted me to a piece of the guidelines I obviously missed haha. Hopefully I have gone through and made the appropriate changes!

Sherlock Holmes was a graduate chemist.

When Thatcher Greene asked him, though in much harsher words, to figure out how he felt about her, it wasn't a particularly difficult question for him to answer. He was well aware that the various biochemical processes taking place in his body were clearly indicative of what some would call love.

Norepinephrine was responsible for his increased heart rate. The loss of appetite and insomnia were surely due to a rise in dopamine. Serotonin would be the culprit of the increased amount of time spent thinking about her. Testosterone and estrogen fueled lust.

That should have been where it had ended for their plan to work. Attraction was all that was required but their bodies betrayed them by moving onto the next stage: attachment.

The oxytocin had gotten to her first but it didn't take long for his to rise as well. Increased Vasopressin levels led to monogamy, an idea that the two hadn't discussed but simply wound up following anyway.

Even though his mind could easily come up with these answers, it would not allow him to call it love. The feelings and actions were the biochemical processes designated to happen. Sherlock felt that since he did not choose to 'fall in love' that he did not: his body was utterly betraying his mind and he could not hold himself accountable for her misinterpretation of that.

He had assumed it would be quite an easy fix: end the relationship and the molecules in his body would end their revolt. But they didn't. They had only begun.

Sherlock hadn't been to the bar since their arrangement began; he hadn't needed to. She was there to listen to his deductions, compliment his violin-playing, satiate both of their biological urges, and manage to keep up somewhat intellectually. They had spent almost two years that way, enjoying not only their time spent together but the time they could spend apart. Neither wanted a romantic attachment; at least not at first.

She'd been gone for almost a month and the sensation he could only describe as pain had yet to subside. It was making Baker Street almost unbearable. Before, if he was separated from her for a bit longer than his liking all Sherlock had to do was text her. Plans would be made and his mind could be put at ease. In all honesty he could still text her but every time he picked up the phone and opened their conversation, his fingers seemed to forget how to type. Or simply refused, he wasn't sure.

He was pondering which it might be, his eyes flicking back and forth between his now empty drink and the blinking line on his phone which the words "I miss you. I think. -SH" immediately proceeded. Sherlock had managed to type the words between his first and second scotches but had yet to press send.

The phone's screen went black after a minute of inactivity and he sighed, almost relieved by the sight. He couldn't give in. As she had so eloquently put it, she felt certain that he would barely notice her absence and that her leaving was giving him exactly what he wanted.

"You said this would happen. We had a good run but we both know there's no going back from this."

"You never know," he had responded with a shrug, trying to keep his expression as ambiguous as possible. "You don't have to go."

"Oh, but I do," Thatcher said, barely looking up at him as she packed an overnight bag with various items of hers that had ended up in his flat. "There's quite a difference between knowing I'll never hear you say you love me and hearing you adamantly refuse to acknowledge it after I've already said it. Unless you feel up to it now?"

She had stopped gathering her things and finally met his eyes. The longer he stayed silent the more tears seemed to well in her eyes. "Right," she began when it was clear he was not going to be speaking. "Well, this was our deal-no attachment. I broke it so I should be the one to back out. Looks like I can take that job in Boston after all."

And so she had. Thatcher had left that night and by the end of the week the rest of her belongings had followed; 221C sat empty again.

Sherlock hoped that finding someone else, even if only for one night, would be enough to fool his system back into submission. A few women had wondered close enough to be considered throughout the night but none seemed worthy of his attention. When asked, he told the bartender that he did not need another drink and paid his tab without another word.

"What in the hell are you doing here, freak?"

He turned towards the voice with a slow roll of his eyes, annoyed at the disturbance. Especially considering it's cause was one of his least favorite people, Sally Donovan.

"Hello to you too, Sally," Sherlock answered flatly, standing from his seat and heading towards the door. The last thing he wanted right now was to be antagonized and that woman seemed to always know the right buttons to press. However she did not take the hint and followed him out into the street.

"Moved on already, have you?" She questioned, anger clear on her face. It was not a new emotion from her but it's strong intensity was. The reaction confused him momentarily until he realized where her anger stemmed from: the woman was friends with Thatcher. Somehow when their paths had crossed at a crime scene they had connected, become fast friends. Sally had not approved of her arrangement with Sherlock and had made her feelings well-known.

"Hardly. Goodnight, Sally." Sherlock did not expect or wait for a reply, instead turning on his heel and walking swiftly in the opposite direction.

The addict sitting at the edge of his mind urged Sherlock to find a dealer and that within an hour everything would be feeling much better. It was a tempting offer. Almost as tempting as picking up the phone and calling her. They were both two things that he had promised to never do again but one of them had to give. He walked aimlessly though he knew exactly where he was in the city at all times. He had grown accustomed to wandering its streets on his own in hopes of finding some relief, some room to think. He thought about their time together, each slow and steady footstep bringing up another memory he'd rather forget.

The first case she helped him solve.

The first deduction he had gotten wrong about her.

The first time they kissed.

Of all the things Thatcher had done, there was always one moment that stood out in his mind above all others. It was the Sunday night after she had moved into 221C. When he heard her climbing the stairs he assumed that it was to ask, politely or not, to stop playing his violin so she could have some peace and quiet. Instead she just sat down on his couch and they chatted briefly:

"Do the police ask you whether someone committed suicide or not on a regular basis?"

"Relatively."

"But you don't work for the police department? Come to think of it, you never mentioned what you did."

"I do consulting work. Mostly for the police but occasionally I take civilian requests."

"Consulting? So…you're like a consulting detective?"

The only one in the world. The title that he so proudly displayed was bestowed upon him by someone who, had you asked him at the time, wouldn't have lasted nearly as long as she did in Baker Street.

"Mycroft," Sherlock greeted stiffly upon entering his living room. He always knew when his brother stopped by for a visit- the heavy brass knocker on the outside door, which usually lay crooked, somehow always seemed to find itself straightened up. The younger Holmes wasted no time removing his coat and settling into his favorite leather armchair.

"Sherlock, you've been moping." Mycroft said slowly, moving to sit in the overstuffed chair across from him but Sherlock said his name sharply in warning before his buttocks could reach the cushion. He raised an eyebrow at his little brother but stood straight and moved to the free chair at the cluttered desk. "I warned you this would happen."

"What do you want?"

"You need to get back to your old self. What better way to do that than by solving a case?" Sherlock did not respond to the offer, choosing instead to stare immovably back at Mycroft. It was the clear antidote for the affliction but the patient was not nearly as willing as Mycroft expected him to be. "You've got to move on."

"And how exactly would you suggest I go about doing that?" Sherlock inquired, not bothering to hide the snide tone in his voice.

"The same way you always do, of course. Solve the problem; answer the question." Mycroft rolled the thick wooden handle of his umbrella between the palm of his hands.

"I can't," he responded after a few moments of silence. It wasn't an easy answer to give but it was true. There were no unknowns to discover or pieces to put together. "The problem I have is with my heart. Logic and reason dictate my behavior exclusively. My heart usually does nothing more than pump blood to deliver oxygen and nutrients to keep my brain functioning properly but it's getting out of control."

"You miss the girl," Mycroft stated, sitting back a bit and crossing his legs as he watched Sherlock. "You must erase her."

"Erase her?" If he didn't know any better, Sherlock would assume that his older brother had gone crazy. Thatcher wasn't just a name written on a chalkboard somewhere. She wasn't photos that could be burned or pages to be shredded. She was not an abstract idea to be disproved and forgotten. She had wormed her way into every corner of his mind palace, constantly reminding him what he had given up.

"Yes. I believe you've done it before, haven't you?" The older Holmes asked, apparently interested in something under his left thumbnail.

"With trivial information from primary school."

"What's the difference?"

Sherlock could feel anger pooling in his chest at Mycroft's assertion that Thatcher was trivial. Still, it was an interesting concept. It was obvious that time was not going to heal his wounds so the next logical step would be to try something else. He wondered briefly if it was even possible. The longer he sat and thought about it, the more probable it seemed. Since Sherlock kept his memories and knowledge locked away in his mind palace, theoretically all he would have to do is find every trace of her within the mind palace and dispose of them. It would have to be a coordinated maneuver, one that Sherlock certainly could not do on his own. "I'll try," he eventually conceded, not bothering to hide the displeasure from his face. "But I'll need help."

"Obviously."

So it was decided. They would begin the following day. Mrs. Hudson would come into 221B while Sherlock was out and remove any physical items that she believed could trigger his memory. Mycroft spoke directly with Lestrade that night about making sure that none of his people ever brought her up. A similar conversation was had with Molly Hooper and a few other individuals at Bart's. Everyone had agreed but none of them had seemed particularly pleased about it. It was with the knowledge that within twelve hours she would be completely gone that Sherlock finally found his way into bed and drifted to sleep. Every time he closed his eyes she was there; if he couldn't see her face, he could hear her voice or feel her skin beneath his fingertips. Even when he could block out those distractions, he could still feel her presence. She was there like a slight weight on his mind, with him no matter where his thoughts strayed.

When he finally drifted off to sleep, his mind found peace but it was short-lived. It felt like no time at all before he was being awoken by bright sunlight streaming into the bedroom. Sherlock felt uneasy and though he could easily deduce it had to do with his plan for the day, he couldn't fathom why it was bothering him. And that was precisely the problem. The man knew the feelings he was having were due to the sudden change in the biochemical molecules in his brain that had been triggered by Thatcher leaving but somehow having this knowledge did not alleviate the situation. He had always considered the rational portion of his mind to be stronger than the part in charge of the baser human needs. Everything could always be explained one way or another except this. His logic was no longer applying and was becoming subject to his emotions.

Sentimental was not an adjective to describe his personality that he was comfortable with. As he dressed slowly, trying to still his shaking hands, Sherlock became keenly aware of how low he was feeling: it was as if he was going to a funeral. Today he would be witness to the permanent burial of what was left of his humanity. There would still be individuals that he would be more fond of than others but only in the sense that those lucky few would be the ones who annoyed him the least or proved to be the most useful to him. Any hope of retaining any of his limited redeeming qualities was lost.

"'Tis better to have loved and lost than never to have loved at all." Sherlock's mind had been drifting, barely aware of Mrs. Hudson beginning her work in the flat around him. He blinked a few times and refocused his eyes on the owner of the interrupting voice. His brother stood in the doorway, one hand shoved deep in his pocket.

"How would you know?" Sherlock asked coldly. He gathered his coat and scarf, muttering a small goodbye to Mrs. Hudson and followed his brother down the stairs. The brothers walked in complete silence, side-by-side. They both knew there would a car waiting to pick them up but it felt good to stretch his legs, breathe in the London air.

"Where is she?" The question had been on the tip of his tongue from the moment he had seen Mycroft after she had left. Sherlock's pride would not allow him to contact her and find out for himself but if anyone could figure it out, it would be his brother.

"Does it matter?"

"Probably not. But I'd still like to know."

"Boston. Did you expect a different answer?"

Sherlock shrugged. He wouldn't immediately jump to the conclusion that she had lied to him but that was before he had broken her heart. It wouldn't be surprising if she had given him false information, hoping to keep as much distance between them as possible. The idea to catch the first flight from Heathrow that afternoon and go after her crossed his mind but he did not cling to it. If he was going to run after her, he should have done it a month ago; he shouldn't have ever let her leave the country. Now there was an ocean between them instead of two floors.

"How are you going to do it?" His brother asked a valid question over an early lunch but he didn't feel much like sharing. So he didn't. Sherlock only spoke when it was necessary, growing impatient with the activity and ready to go home again. The question voiced itself again once he was safely ensconced inside Baker Street. Mrs. Hudson was downstairs and Mycroft had left not long after their return.

He would start at the beginning and work his way through, gathering the memories one-by-one and removing them. It was the only way he could be sure that every trace of her would be gone. His mind would take the blanks and fill them in easily, creating a Thatcher-less existence. Once comfortably settled into his favorite armchair, his eyes closed slowly and did not open until he heard a feminine voice.

"Why couldn't you have just said it back?"

Sherlock felt a twinge of pain, practically flinching at the sight before him. He was still in his living room at Baker Street but there were stacks upon stacks of photo albums filling nearly every inch of normally empty space on the floor. The one thing he had been hoping to avoid sat across from him in the overstuffed armchair, legs crossed comfortably beneath her and settled comfortably into a reclining position. When he didn't immediately reply, Thatcher raised her eyebrows at him questioningly and sat forward a bit, trying to encourage an answer out of him.

"You know why," he replied sullenly.

"No, Sherlock, I don't. You love me. Why can you not just accept that?"

"Because it's not true. It can't be."

"So here sits Sherlock Holmes, so infallible even he can resist falling in love?"

He scoffed but did not otherwise reply to her statement. He took a moment to observe his surroundings, unsure of what lay scattered about the room. "Why are you here?" he asked, desperately attempting to change the subject.

"I'm not," Thatcher answered with a shrug. When he met her eyes, he knew exactly what she meant but asked her to explain anyway. "I'm not Thatcher. This is simply the form your humanity has chosen to take. I won't go without a fight, Sherlock."

"I just want to forget her."

"Why?"

"It hurts."

"That's not a bad thing," the girl answered with a shrug. "Pain and sentiment are not weaknesses, Sherlock. They are a part of life and mostly unavoidable. It would be better to face them and come out the other side than simply ignore them."

"I tried."

"Look, Sherlock. I'm not here to do anything except guide you. If you have already made up your mind that this is what you're going to do, then I can't change that." She unfolded her legs and somehow found a space on the floor for her feet and stood without even the slightest hesitation. Her eyes scanned the room for a few moments, searching for something. He knew immediately when she had found it, her eyes lighting up like they had when making breakthroughs in her research. She stepped purposefully, calculating each place she could put her foot before doing so. The album she was looking for wasn't more than three well-placed steps away and once it was within her grasp, she leaned down and picked it up gingerly. As she straightened back up, holding a one-inch photo album to her chest, all the others in the room dissipated into thin air, as if they had only been an illusion.

"Are you ready to begin?" Not-Thatcher asked, her head tilted slightly to the left as she watched him for an answer.

"Yes," he answered quietly, managing to keep his tone as level as possible.

Thatcher moved towards him quickly, his mind barely having the opportunity to realize what she was doing before it was already done. She sat easily on the right arm of his chair, her cold bare feet finding their normal spot in the seat, right next to his body in an attempt to gain some warmth. She leaned back, her left arm resting against the back of the seat. It was how Thatcher had always sat when reading, usually one of his cases or the newspaper, over his shoulder.

"Here you go," she said sadly, placing the photo album on his lap. It was much heavier than he had expected it to be. He let his hands drift over the worn leather binding. Though he had never seen or held it before, it felt familiar. He took a slow, long inhale before opening the album.

There were three photos on each page and lines stretching out from the right side, apparently to leave an explanation of the event or date: the first time they met; every case she had ever offered help on; the nights she had patiently listened to him mumble on for hours aimlessly; the time spent peacefully in silence as his mind turned over various ideas.

Everything was there. He attempted to keep the emotions from his expression, not daring to look up into what he remembered to be Thatcher's eyes. "What do I have to do?"

"You could burn them," Not-Thatcher answered with a shrug. That wasn't a particularly appealing option to Sherlock. Apparently sensing his unease, she continued, "You have to destroy them. It's the only way to truly rid yourself of the memories."

Every passionate moment; the few times after said moments sleeping peacefully next to one another; comforting her when she became overwhelmed by the stresses of research; and meeting her parents.

"Is it permanent?" He eventually asked, slowly turning the pages.

"Yes."

The first of her panic attacks he had witnessed; carefully cleaning various scrapes she acquired when mugged once on her nightly run.

"How will I know if I am successful?" He asked, finally looking up into her face.

"You won't. You'll forget there was even anything to rember."

The first time he made her cry; when she admitted that she loved him; the pain in her eyes when he didn't say it back.

As he got closer to the end of the album, the quantity of happy memories decreased exponentially. The last few featured scenes from their big fight, the end of them, and Thatcher leaving.

"What if I'm not ready to let go? Permanently, that is. Do I have any other options?"

Not-Thatcher looked troubled upon hearing his question. "Yes," she answered slowly, conflicted on whether she should be telling him. "I can try and hide them from you. But I can't promise it will work. If you search long and hard enough, you may find them again."

Sherlock closed the album, his hands shaking minutely as he held it out for her to take from him. She accepted it with a nod and stood to leave. He stood also, instinct telling him to follow her, but her hand resting momentarily on his shoulder was enough to stop him. She shook her head lightly and he sat back down defeatedly. She made it to the door at the top of the stairs before he asked his next question.

"Will I see you again?"

"I suppose, if you want to. When you're ready to remember, you will."

"What about you?" Sherlock asked, gesturing towards her form.

"Oh. Me? No. Not anymore. I have to keep this hidden from you. We both know that will be a full-time job. From now on when you see me, if you do, I will be in a different form than this one."

He seemed thoughtful for a moment and she waited to leave so she could hear his next words. "I will see you again," he declared with a small, single nod of his head.

"Well, I'll see you then," she answered, the side of her mouth turned up into a smile. Not-Thatcher turned and left the room, disappearing down the stairs.

Sherlock recognized the sound of her footsteps descending the stairs, having heard it countless times before, but the further down she got, the fainter the sound became, less familiar. There was a very fleeting moment of fear as he realized what was happening but after another had passed, the fear was gone as was the explanation of it. A sense of content washed through him: the pit in his stomach had finally filled, his hands had calmed, and there was no longer a dull ache throughout his limbs.

It had taken him two years to learn her, yet only two seconds to completely forget.

"Sherlock," a voice called out, bringing him out of his own mind. His eyes had been fixed on the right arm of the overstuffed armchair and they moved quickly to the source of interruption.

"What are you doing here?" He asked Mycroff, not bothering to hide the snide tone from his voice or the confused look on his face.

"There's a case I thought you might be interested in," Mycroft replied evenly, apparently not noticing his brother's contempt.

"Is that your way of asking for help?" Sherlock asked, his fingers slowly steeling under his chin, a smirk on his lips.

Mycroft grimaced, not wanting to let the obviously insulting question slide but this would be his brother's one reprieve. Instead, he turned to deducing whether his brother had been successful. "Has Mrs. Hudson found anyone to take the downstairs flat or have you scared them all off again?"

"It's been empty of nearly three years," he answered without the slightest hesitation. "I assumed she gave up on the venture ages ago."

"So will you come? You should only be gone a month or two, depending on how quickly you work."

Sherlock pondered his options, observing his brother carefully. Though doing anything Mycroft requested was an utterly repulsive idea, he realized how bored he had been lately. The challenge set forth was enticing and he could not say no. "Oh, why not. It did always make Mummy happy when we played nicely with one another."

The boys smiled, each annoyed at the other, as Sherlock stood from his seat and began back towards his bedroom. "You won't need anything. It will all be provided." Mycroft called out.

"Have it your way," Sherlock replied with a heavy sigh as he returned to the living room. He began pulling on his coat and scarf, and then to follow Mycroft down the stairs but something stopped him. He turned back and reached for the door handle, pulling the usually-open hall door shut behind him. There had been many occasions before that he had run out of Baker Street without so much as a second thought but something felt different this time. He couldn't place the feeling so he tried to put it out of his mind, planning to examine it further at a later time. "Goodbye then," he said quietly, as the door shut and the feeling of the lock falling into place reached his hand.