A/N: Well, here it is. As soon as the promo was released and I heard Barry say that he thinks Cisco and Caitlin might be working for Wells, also knowledgeable of his secret, my mind thought But why not?

So here is some absolutely canon-divergent AU where Caitlin becomes Killer Frost and Wells is the Reverse Flash and they are a rather villainous couple.

Disclaimer: I own nothing, as per usual.

This is how it begins:

When Caitlin Snow first meets Harrison Wells, it is at a conference for bioengineering and it takes him no more than ten minutes of conversation to convince her to get a drink with him. She is 22 years old, a prodigy, just months – and some serious convincing – away from receiving her PhD, but nothing feels quite as good as having the head of the best research facility on the East Coast be interested in her.

It takes him two hours to convince her to work for him and thirteen minutes from the bar to her home for Caitlin to decide that there is so much more she wants to be doing with Harrison Wells.

When he doesn't make a move to leave her instantly but instead takes off his glasses and just looks at her, she knows that she is not the only one who is entertaining some highly inappropriate thoughts.

So she kisses him and when they end up in her bedroom and he presses her into the bed, she is extraordinarily glad that none of her rational thoughts are even vaguely around to stop her.

It's only later when he's idly caressing her spine, counting the birthmarks which curve upwards in a constellation of their own, that the words escape her in a quiet whisper. "I was really looking forward to working for you."

His fingers stop for a moment and her heart beats eleven times before they resume their path and he answers. "With me, Caitlin. I don't see why you can't still do it."

She smiles into the pillow.

And this is how it ends:

"What do Dr. Wells and the Reverse Flash have to do with each other?" She stares at Barry who is more agitated than he has ever been and her heart thrums in her chest; she wonders if he can hear that as well.

Cisco is beside her, looking less shocked and more frightful of what might come out of Barry's mouth, frightful like he knows exactly what Barry is going to say.

"They're the same person."

Everything shatters around her.

Barry proceeds to pull down something resembling an evidence board and among the hundreds of papers, she sees pictures of Wells, notes, data and it all jumps around a bit in front of her eyes. She does not understand.

"What are you saying, Barry?"

The look he gives her is sad and angry and devastated and he takes a deep breath before opening his mouth. "Dr. Wells killed my mother."

She feels like the air is collapsing in her lungs and there are no words she has to offer in return.

Cisco is less awestruck. "But we proved that he wasn't in the house that night, Barry! It was your blood."

He shakes his head and starts pacing in front of them, gesturing wildly with his hands and not really looking at either of them. "That doesn't matter. Look, I know that you both think he's a great man and so did I but he is not! Simon Stagg, Wade Eiling, Mason Bridges – do you know what they all have in common? They had dealings with him and now they're all missing! Look at the facts! Harrison Wells came here after my mother's death and after his wife was killed in a car accident. Before that, he barely existed. There is nobody who even quite knows him from before STAR Labs and the few that do say that he changed after that accident."

"Wouldn't you change if someone you loved was killed right in front of you?" Caitlin finds her voice again and tries to find the logical fallacy in what Barry is saying.

"No, you don't understand. I don't think it's even him. Nobody knows anything about Harrison Wells more than fifteen years ago. And how did he build the particle accelerator so quickly? How does he know so much about everything?"

"He is a genius, Barry!" Cisco butts in but there is no real conviction in his voice.

"Or he is not from here."

"You don't mean Central City, do you?"

He seems to contemplate his next words for a moment. "I mean, if I can run fast enough to travel back in time, and if he is the Reverse Flash, it is more than possible that he's done that. And for some reason, he is now here."

Cisco looks like the new information is making sense to him but when Barry turns his eyes towards her, Caitlin just can't say anything.

"I need some air."

She leaves his workspace before he can interject and doesn't feel like she's getting enough air into her lungs before she's far down the hallway.

Everything Barry has just said rings in her ears and she knows what she must do, has known since he revealed his theory that this is what can be done.

She pulls out her phone and presses the first number on her speed dial, closing her eyes briefly as she waits for the call to go through.

"Hello?"

"They know."

She can hear him breathe deeply into the phone and there is silence for a moment. "Are you safe?"

She nods and collects herself. "Yeah, yeah, that is…it's just you."

"Are you sure they don't suspect you?"

"I'm fine for now."

"I'll figure something out, Caitlin."

She lets herself believe in that for a moment, lets his words and his voice sooth her fears which have arisen. "I know you will, Harrison."

After all, that is what they have for now.

But this is what happens in-between:

He doesn't look at her any differently when they're in the lab and neither does she. She is not some silly girl who can't contain what happens between them in the privacy of their respective homes, so she works with him, oversees the various projects that need to be dealt with and always addresses him as Dr. Wells, never anything more or less.

And if he sometimes calls her into his office to discuss future ventures, takes off his glasses and intently stares at her until she crosses over to his side of the desk and they proceed to be decidedly unprofessional with each other, it really isn't her fault.

Still, there is no preferential treatment thrown her way – which she doesn't want, thank you very much – and his eyes only linger on her a tad bit too long when he thinks no one is watching.

So she really has no clue what is going on when he suddenly calls it off a year into whatever they were.

"I don't think it is very professional, Dr. Snow."

He has never called her Dr. Snow in private before and it stings more than she thought it would, the fact that he's actively putting space between them and trying to distance himself as much as he can.

"Has it ever been professional, Harrison? Why now? What has changed?"

His gaze is piercing but there is a reservation there, a shadow of something she's only caught glimpses of before and she doesn't understand what he is trying to tell her without saying the words outright.

"I've been misleading you if you thought this was going anywhere more than what it has been so far and I do apologize for that. As for now, I propose we continue our professional relationship and nothing has to change."

He says it so calmly, in a manner he has never shown her before and it frightens her how much of him she still doesn't know.

"You mean let's be friends?"

She lets the sharpness bleed into her voice, lets him know that he is not the only one who can be callous and cruel with words and the flicker of hurt in his eyes gives her a kind of satisfaction.

"No. We'll be what we've always been – supervisor and employee."

Caitlin has forgotten that challenging Harrison Wells with words has never ended well for anyone. Still, she holds her head up high and nods once. "If that's what you want, Dr. Wells."

Two weeks later she sees Harrison in deep conversation with Hartley Rathaway, the new prodigy who wastes no time in telling everyone exactly that, and when he leans closer to touch his hand to Harrison's back and he doesn't move away, she understands.

The surge of pain is totally uncalled for.

"Hey, is everything alright?"

Almost out of nowhere, Ronnie Raymond, the new engineering hire pops up next to her and looks at her with something akin to worry in his eyes. She's long known he has harbored a crush on her but what did that ever matter before?

Now, though, she takes a moment to appreciate it and finds herself leaning closer to him. "I'm fine, Ronnie," and bestows a blinding smile on him. She contemplates what she's about to do for a second before it crystallizes into a plan. "But how about we have drinks tonight and I can tell you all about it?"

His mouth falls a tiny bit open and he must not have expected this at all but he recovers quickly enough and smiles. "Absolutely."

She does not turn around to see Harrison's eyes glued to them, or the flash of red lightning that goes through them when she laughs at Ronnie's joke as they walk away.

She might've started it as a way of comeuppance, as a catty and childish way of making Harrison jealous but she finds herself falling in love with Ronnie, almost with no resistance. He is sweet and caring, always brings her coffee in the mornings, takes her on spectacular dates, and makes her feel like he has never seen anything more special than her.

There is an unspoken code of conduct at STAR Labs that scientists from different departments are not prohibited from seeing each other, and Harrison doesn't say anything about her and Ronnie when they publicize their relationship. It feels amazing, to be loved like this, openly and honestly, to be able to smile at Ronnie the way she wants to, and to lean into his touch when they're in public.

Hartley is more insufferable by the day, and she finds it especially pleasurable when Cisco Ramon, the new engineer holds his own against him. She is pretty sure that she is the only one who notices that Hartley sometimes comes to work, smelling faintly of sex with Harrison – and she should know, the smell will probably never leave her olfactory memory or her dreams – and wonders if Hartley is better at compartmentalizing and if he can offer something better than she ever could.

When Ronnie asks her to marry him, she is frozen for a moment, looking down into his hopeful face as he kneels in front of her, with the most gorgeous ring she has seen between his fingers. The air around them smells of fall and promises and she allows herself a second to contemplate what would happen if she said no, if she didn't take the family he is offering her, the years ahead.

She says yes because she can see the perfect future with him.

Definitely not because the alternative still makes her heart beat a tad too fast.

Harrison offers his congratulations without batting an eyelash, no emotions detectable on his face, not a single snide comment about how quickly their relationship has evolved, merely smiles not unkindly.

When their impromptu engagement party is in full swing, she finds him in his office, already working on some documents. "You left pretty quickly, Harrison."

She feels like getting a rise out of him, wants to see the reaction some part of her hopes is still inside. She hasn't called him Harrison since he ended things.

He looks up at her but still without betraying any emotions. "I had some thing I had to take care of, Dr. Snow. But I hope you'll be happy together."

The words might not be meant as a challenge but they strike her still, and she engages in a fight she probably should not have started. "Why don't you ask me if I am? Happy, that is."

He is more amused than anything but gives in. "Are you happy?"

She smiles openly, wantonly. "Yes, so very much. I hope you're as happy with Hartley."

Now something akin to surprise and anger plays on his face before it's a cool mask again. "That is really not comparable, I think."

"Why not?"

She can feel the danger in her question, the danger if he should respond truthfully and the recklessness she feels in her veins makes her hope he will.

"I am not doing this to get back at you."

The implication maddens her; how dare he insinuate that she doesn't love Ronnie, that their relationship is all about him. "I do love him, you know."

"Of course you do." But his voice is derisive. "That's why you've come to me, because you love him so much. Just what kind of response are you expecting from me, Caitlin?"

The sound of her first name from his lips washes over her in unexpected ways and she's been waiting for it for so long but instead of fueling her anger or heat, it just brings a wave of sadness and she decides to speak the truth, for once. "If it wasn't for Hartley then why did you end it?"

He sighs and she can sense his reluctance in giving her an answer until he takes off his glasses and looks at her again, just like he once used to. "It had to be done."

A noise from the hallway breaks the tense atmosphere between them and she realizes that it's her engagement party and here she is, expecting some kind of epic declaration from another man who doesn't even want her.

She clears her throat and steels herself against his gaze. "I hope it was worth it then."

When she turns around and leaves, she strains her ears as much as she can to hear whether he responds something, even under his breath but there is only silence behind her. She doesn't know if it infuriates or pleases her.

When the particle accelerator explodes and Ronnie goes to his death, her life is turned upside down again. She wanders in her apartment aimlessly, not eating and not sleeping because she can still hear Ronnie's parting words, can feel the energy from the explosion wave washing over her, and the sound of a falling beam breaking Harrison's back.

The only reason she leaves the apartment is to go and visit him in the hospital, reading to his sleeping form the newest scientific articles. Despite the bad blood between them, he is still her friend, and whatever more he was, she can just not think about that. When he comes to, she keeps the newspapers away from him, and chats about the most inane things, all to stop thinking about what really happened.

Of course, he finds out eventually and the seriousness with which he offers her a way out of STAR Labs is uncanny. She pretends to consider it but knows that she could never leave the lab, the work, could never leave him, has not really let go of him anyways.

So she stays and together, they find Barry Allen, along with Cisco, and this feels like the right thing to do.

One night she's alone in the lab, looking at Barry's sleeping form and idly twirling her engagement ring around her finger when she decides to take it off, just for a second.

Her finger feels bereft without it but that is not all. Suddenly, a wave of cold washes over her skin, penetrating deep within her and with astonishment, she realizes that she can feel ice crystals in her palms and the air she breathes is suddenly cold like the Arctic, despite being inside the lab.

She tries to get up and call for help – though who could help her here? – but her hands tremble violently and she merely manages to rattle the metal desk next to her before the cold encases her everywhere and she loses consciousness.

When she comes to, there is a strange buzzing in her veins, akin to small vibrations and when she opens her eyes, Harrison is right there in front of her, and she realizes that he is holding her by the shoulders.

"Wh-what are you doing?" She whispers meekly as she recognizes that for them to be in this position, he cannot be in his wheelchair and that just isn't possible.

He doesn't relinquish his hold on her but his eyes rove over her face, as if in search of something. "Do you trust me?"

And that is the one thing which has always been true so she nods.

His face relaxes a bit and he even smiles. "Good. You're going to be fine."

She drifts off again because when she wakes up the second time, she feels like herself. She also notices that they've moved from STAR Labs to Harrison's home and she's lying in his bed. Everything still looks like it did the last time she was here.

When she gets out of bed and goes to search for him, her feet instinctively take her towards the glass-ceilinged area and there he is, standing, his back towards her.

It takes her a few seconds to understand the picture in front of her. "You…you're not in your chair."

He turns – and he must've known she was there – and something like hesitation and longing war on his face. "Caitlin. I can explain."

"Was it the wave of energy? Did it change you?" She thinks back to whatever happened in the lab and already knows the answer to her next question. "Did it change me?"

"It changed you." He comes towards her until he's close enough to touch and takes her hand into his, swiping his thumb over her engagement ring. "I think the energy got stuck here, in the ring, and that's why you didn't show any signs before you took it off."

She doesn't miss that he didn't answer her question about him, and guesses that the answer must not be as simple.

"And what happened?"

"Your body has adapted to the temperature and for some reason, you are now attuned to the cold. You cannot control it yet but I can teach you."

"How? How did you stop it?"

He turns serious and lets go of her hands but doesn't move away. "I…vibrated your cells at the right frequency that they would produce more heat than usual and raise your core temperature before you would freeze over. And then I put the ring back on."

And just like that, his hand blurs in front of her face and she stares at it, understanding but not quite believing. "What are you? What am I?"

"I prefer to think of it as being enhanced, Caitlin. We have been given more and we should really take full advantage of that, don't you agree? Wouldn't you like to learn how to control your powers?"

His voice is silky smooth now and his fingertips caress the length of her arm and she shivers; her body has not forgotten what it felt like to be touched by him. And there is something stirring inside her, answering the siren call of his words, fighting to break out of the hold her mind tries to exert over it.

"I know you're meant for this, Caitlin. But you have to make the choice. Do you want to know what it could be like, to be so powerful that no one could stand against you? Do you want to be in control?"

She imagines what he is offering, imagines being able to really use the power she had felt in those moments before she passed out, to not let them be in charge of her and the darkness explodes in her chest.

He is looking at her so expectantly, and she wonders if his alternative plan was to kill her when she said no – it makes sense, she would only be a liability in any other scenario – and so she smiles.

"Show me." She whispers it and before he can respond with anything else, she kisses him and the power that surges upwards inside of her is exhilarating as he responds with equal enthusiasm and she feels like they are just steps away from something great.

It takes them two tries before they make it back to his bedroom but that is plenty enough to make her realize that if she thought he was a good lover before, it is nothing compared to the headiness of their reunion now. There is something unmistakably better about being with him like this, everything open between them, everything much better, much more real.

When she comes down from her high, she regards him intently and wonders if he would tell her the truth if she asked. "Why me? How did you know I wouldn't just leave you?"

He is quiet for a few moments but she doesn't say anything else until he turns towards her again. "I just had to believe in it, Caitlin."

The next six months, he teaches her how to control her powers, how to call upon them and how to not let them consume her but direct them against everything else. The destruction she can bring forth is attractive and when it's right within her reach, she feels utterly unbeatable.

In addition to that, Harrison tells her the truth about so many things, about his real name, about Barry, about speed, and lightning, and time travel, and she takes it all in stride because it is all merely a part of him and she is now a part of him too, inextricably so.

"I love you." She whispers it into the space between his shoulder blades one night. He stays utterly still in her embrace before turning around and facing her, his features solemn and almost sad.

"Oh, Caitlin. You know that to me, you've been dead for centuries."

And of course, she knows that the incompatibility of it all is staggering but the words have been pulsing inside of her for too long to not be spoken out loud. "I just wanted you to know, that's all. You don't have to say anything."

The forlorn expression on his face makes her realize that it's not for lack of feelings that he has never said those words to her but rather the opposite.

"When I look at anyone else, all I see is like dust, like remnants of an old film because I know that they are actually dead, that they are not real. But when I look at you…" his voice trails off and her heart feels like pounding out of her chest, waiting to find out if he'll continue or not.

"When I look at you, you are so impossibly alive and I know that this is the worst thing I could do to you but…I do love you."

Despite the warnings it should give them both, it only serves to make her happier than she has ever thought she has the right to be.

When Barry finally wakes up, Harrison forbids her from using her powers anywhere, oddly content to keep her safe but risk his own identity again and again, over Stagg, over the meta-humans, almost coming close to revealing himself to Barry. She berates him when they're alone but he can only see the future Barry's powers are supposed to bring, the promise of the speedforce which would eventually transport him back into his time.

He seems to slip slightly away from her and she wonders if he is trying to untangle the mess of feelings they've created, in expectation of returning to his time and leaving her behind. It hurts her to think that but she knows that it makes sense; he can't possibly bring a long-dead girl with him into the 25th century.

It is absolute foolishness to think that just because he loves her, he would risk trying to speed through time with her.

Ronnie emerging from the dead makes the impossibility of their situation even more painfully clear. They don't talk about it before they're camped outside of Clarissa Stein's house, waiting for Ronnie to emerge.

"Why would he come back here?" Caitlin wonders that out loud, mostly to break the stiff quietude of the van and the awkward tension that seems to be seeping in.

"Because this is his home. I don't mean the actual house, I mean Clarissa. She's his home. And we all want to go home again. Where we feel safe, where we feel loved."

Not for the first time, she wonders if his words have a double meaning, if he's actually talking about his real life, and the people who might be expecting him there, people who are not dead to him the way all of them are.

"Ronnie's your home."

It infuriates her to no end that he would choose this moment to bring up what he hasn't wanted to do so far, to say things like they don't really matter, like he doesn't even care.

"Not anymore. What do you expect will happen after this?"

His gaze is calm and collected as he regards her carefully. "Well, obviously we have to wait for him to appear first but we'll have to bring both him and Martin Stein home. My work is what did this to them both and I have to fix it."

"That is not what I asked. What do you expect will happen to us?"

A muscle ticks in his jaw as he looks away from her, at anywhere but in her direction. "He is still your fiancé."

"Do you want to know if I still love him?" She is angry at his impassiveness and tries to get some sort of a reaction out of him.

He doesn't say anything, so she continues. "He is not my home. Stop pushing me away; you know that I love you."

Before he can say anything else, there is a flash across the sky and they need to go deal with Ronnie but his words still echo in her head, and she lets his dismissal guide her actions, lets herself pretend that she's still the same Caitlin Snow who was happy to be Ronnie Raymond's fiancée after he is separated from Martin Stein. She lets Ronnie kiss her and hug her and love her but all the same, her heart aches for what she truly needs.

In the end, letting him leave for Philadelphia is an easy decision, as is driving straight to Harrison's house and letting him know in no uncertain terms that he would do better to think that he can just cast her aside because he thinks he knows what is better for her.

"We are in this together, Harrison. Don't ever presume that I'd be better off without you or that you somehow need to offer me things I don't ask you for. If I tell you that I love you, then believe it."

He holds her very close that night, whispering apologies and words of affection into her hair, her mouth, her skin and she lets herself believe that he just might be willing to accept her not leaving him, no matter how everything will end.

Because she is so consumed with wondering how all of this will come to a head, she pretty much misses when it does exactly that. When Barry reveals his murder board, she is genuinely surprised but mostly because he was not supposed to figure this out and she cannot understand where all of this will leave them.

The phone call appeases her worries somewhat but still, Harrison has no specific plan in mind, tells her nothing of what he will do.

And when Barry asks for Felicity and Ray to help, and later Oliver, and then even Ronnie and Martin Stein, she feels acutely that the end is nearing and she doesn't know how to stop it from happening.

Ronnie keeps a close eye on her, too close but she shies away from his touches and his love, and in the ultimate chaos where they all try to find Harrison – who has disappeared – he doesn't seem to notice how she flinches whenever they talk about killing him.

She almost hopes that he has managed to make the speedforce work for him, that he has made it back into the 25th century somehow and is safe.

And then he shows up outside of STAR Labs and all hell breaks loose. There is a tense stand-off which he and Cisco watch through the monitors, Harrison almost unrecognizable in his yellow suit but she can feel that it's still him.

Her own powers are calling to her, singing in her veins, urging her to go out there but she fights it, knowing that he would not want her to expose herself unless she really has to; that has always been his directive.

But when she sees his speed fluctuate, she knows that he still hasn't stabilized his suit enough to take on all of them. She looks at Cisco, immersed in the screens and Joe who is not paying any attention to her and before they can notice her distress, briskly walks out of the lab.

Once she's outside, Barry turns to look at her, as well as Ronnie and they both seem to be utterly dumbfounded at her appearance.

"Get back inside, Caitlin, it is not safe!" Barry yells at her and gestures for her to leave but she finds Harrison's eyes instead – even underneath the glowing red orbs, she always knows exactly where they are – and takes off her ring. The ice surges through her and she lets it consume herself before moving to Harrison's side of the fight.

"What are you doing, Caitlin?" Barry's eyes are impossibly large now as he takes the unexpected picture in and she only casts a quick look at Ronnie to see the heartbreak and disbelief on his face.

"I am where I'm supposed to be." The words come to her, unbidden and they convey everything she needs to say to Harrison without uttering anything else.

She is making her stand, right next to him, the 25th century and her needing to keep a low profile be damned.

They are afraid to attack her, she can tell because all of the power is directed straight at Harrison but once she ices the ground beneath their feet and short-circuits Ray's suit with a blast of cold, they realize she is not there to joke around.

Harrison blasts around at remarkable speed and she can barely make out him and Barry and Oliver, as they chase each other, leaving her to fend off Ronnie – who is still too cautious for his own good – and she easily counters his fire with her own ice.

"Caitlin, what are you doing? This is not you!" He pleads with her in between the attacks but she can only think of Harrison and the danger they're both in and has no patience for people telling her who she is or isn't when she knows perfectly well that the song in her veins is exactly who she is.

She falters for but a second when she sees Harrison's powers fluctuating and his speedforce leave him and that moment is all it takes for Oliver to trap her and wrap a rope around her, and Barry to put her ring back on her finger, containing her powers.

She struggles against it but it only tightens the trap and she is helpless to watch as the two speedsters chase each other, until she can feel Harrison's eyes on her and the exact moment he notices her predicament because that is when he and Barry come crashing to the ground.

His speedforce is depleted and as Barry pulls off his mask, his eyes meet hers and the sadness in them pierces her heart.

"If you stop fighting, I promise you, we will help her be better." Barry says it with utter conviction, as if he thinks Harrison has her under some kind of spell and oh, how foolish can one boy be.

But instead of saying anything to the contrary, Harrison nods. "Fine."

They escort him into the pipeline and he doesn't say anything else; Barry gestures for Oliver to take her into the main lab instead and stays there with her.

He stares at her as if he doesn't comprehend; he probably doesn't.

"Did he do this to you, Caitlin? Did he threaten you to help him?"

And she can see the way he wants to believe that she is good, that this is all some kind of a misunderstanding and that she'll maybe crumble and expect Barry to be the hero who saves the day, who gets to save her. Still, he keeps a close eye on her ring, and would probably manage to stop her before she gets that off again.

"If he made you into this…this thing, I promise you that we will figure it out and we'll help you be better."

Caitlin understands exactly why Harrison let himself be taken, why he acquiesced and let Barry think that she's under his influence because if she told him just that, he would believe it because he wants to believe only in that truth.

So she could say just that. She could look at Barry, maybe even cry, and tell him that she doesn't know who she is, that Harrison Wells is in her head and Barry would do anything to help her. She could betray Harrison and save herself.

Caitlin's eyes well up, just a bit. She lets a tiny sob escape her and Barry latches onto it like he's been waiting for her to crack.

"I can't breathe, Barry." She gestures to the rope and he hesitates for but a second before untying it quickly and efficiently. She breathes again and smiles at him, still teary-eyed.

"It's okay, Caitlin, it's okay."

"Barry?"

"Yeah, Caitlin?"

His attention is focused solely on her face and he is eager to hear her apologize or completely break down and that is all what she needs to slip off her ring again.

Her hands cup his cheeks and she lets the iciness wash over him, watches as his skin turns paler and he chokes on his breath.

"I'm sorry, but to me, you've been dead for centuries."

When he passes out from the shock of cold, she lets go, knowledgeable enough that she can't kill him because Harrison will still need him in the future. Instead, she accesses the pipeline controls and opens the door to his cell. He's by her side in a moment, the speedforce back in play and he looks at Barry and then her, not understanding for a moment.

"I gave you an out, Caitlin."

She smiles and steps closer to him, shaking her head as if he's said something incredibly stupid.

"And I'm right where I'm supposed to be."

So this is how it really ends:

They walk – speed – out of STAR Labs, hand in hand. The 21st century is theirs for the taking.