He knows as soon as the ends of her hair brush his cheek, her whispered you have no idea a shiver in his ear - he knows more than just that he wants to follow after her to make her accept his offer, more than the fact that she is one of the biggest teases he's met in a while (oh, how that will just be cemented in the months to come) - he knows clearly, unmistakeably, that he'll fall for her.

It is not a question of if, only one of how.

So he watches, and he waits.

And theorises.

"I thought you were giving up on asking me out?" Beckett leans back in her chair and throws down her pen, crossing her arms. He wants to cower at the height of her eyebrow.

"What kind of man would I be if I weren't trying to get into the pants of a beautiful woman I work with?"

"Oh I don't know, a considerate one?" She rolls her eyes, picks up her pen, flicks it through her fingers. "If you're going to sit here being lewd will you at least go and get me a cup of coffee so I don't feel like pulling my gun on you?"

He's out of his seat before she can finish her sentence and back before she can register he's even been gone, the coffee cup plunking onto the desk in front of her.

"I've been thinking."

"About what?"

"What'll happen if we go on a date."

She stops scribbling down notes, and fixes him with a look that clenches his gut, her pen still poised on the paper.

"Picture it, a tiny Italian restaurant on the outskirts-"

"No. For the love of god, Castle, be quiet for five minutes."

This is how it begins.

In his head he calls it The Unavoidable Coupling of Kate Beckett and Richard Castle. Never out loud, of course. But he knows there's a pool and pools always need names, right? Even if he's the only one who knows what it's called. It's mostly because he's not going to risk Lanie finding out and telling Beckett that he's encouraging bets on something that is never going to happen (her words, obviously).

"When you touch his jaw, does it cut you?"

Even on the other end of the phone he knows she's rolling her eyes at him.

"Don't get jealous, Castle. What do you want?"

"I'm bored." He doesn't add that he is a little jealous, and more than a little annoyed that he has to sit at home while Beckett's ex-boyfriends parade their far-too-square jaws around the city. With Beckett.

"Can't you call Ryan and annoy him? The only thing I've seen him do this afternoon is read one of your books and drink about six espressos. You could both sit around and theorise all you like about the facial structure of your colleagues, and would mean you wouldn't have to irritate me."

"Ooh, do you want to hear the new theory I have about-"

She hangs up on him.

After they watch the Russian mobster get bundled into the back of a cop car, he turns to her and can't help but say "You know, we should really go undercover more often."

"If you don't stop checking me out I'm going to steal Esposito's gun and shoot you somewhere that'll guarantee Alexis remains an only child."

He winces. Then he sidles up to her. "Are you sure you wouldn't rather put that accent back on and make Richard a lucky, lucky man?" He waggles his eyebrows and he knows she's torn between wanting to laugh and wanting to make him bleed.

On the drive back to the precinct he watches her face (well, his gaze does occasionally dip to her neckline or the height of the cardigan covering the tops of her thighs, but still – mostly her face).

"Stop it."

His eyes snap to his side window. "Not doing anything."

She lets out a disbelieving hmmph, then draws in an audible breath. "Spin a theory for me, Castle. What is it about undercover operations that makes men so giddy?" Her eyes glitter with the lights of the city when she looks at him, and he smiles almost tentatively.

"You don't think it's even a little bit glamorous? You don't like the danger of it?" He rakes his stare from her legs up to her eyes.

"The danger is what makes them so risky, Castle. It may look fun and glamorous in the movies but in real life it just means the chance that things could go wrong is even higher."

He contemplates. "I guess it's a good thing I've got a very convincing Russian," he pauses, weighs up the word threatening on his tongue, "lucky charm, around then."

He steals a glance and is pleased to see an amused smile widen her mouth. Beats a punch in the eye any day.

It takes her a while to forgive him of his sins, which he supposes is understandable. And for a while she won't even let him say the word theory without a threat to some part of his anatomy he'd rather not lose (which is all of it).

The first time she doesn't say anything about it is when he speculates about the likelihood of her being able to fight off a shock collar in the nude just like her alter ego.

She does walk off scowling when he asks "Would you like to test it out? We could use my apartment. Film it, maybe."

"They have sex after their dinner at Remy's. It's not entirely clear when it became inevitable, they simply share a cab and somewhere between him telling the driver her address and the first orange light they run she's kissing him like if she stops she'll die - or come to her senses and push him out of a moving vehicle, one of the two, I guess. By the time they get to her place she's straddling him and he's in the middle of cursing her for getting changed out of that extremely flattering dress because now he can't find the damn zipper on her pants..." his whisper close to her ear drifts off, and he sees the clench of her teeth in the shadows.

"You know, this story relies pretty heavily on me letting you feel me up in the back of a cab, Castle."

Their current cab driver looks in the rearview mirror, eyebrows disappearing into his hair. She holds back a smirk, if only for the sake of class.

"It'd also blow away your whole 'we're not romantically involved' thing you've got going on. Speaking of blowing, do you want me to continue my story?"

She just glares at him.

He barely notices a year has passed. He barely notices he had a life before this one.

Her apartment blows up and there are no second thoughts when he says yes you do and convinces her to stay with him. There is only making sure he doesn't feel his stomach plummet the way he did earlier as bright orange light burned before his eyes. There is only making sure she's safe.

He watches her walk up the stairs, and wonders if she'd object to buying him buying her the available apartment two stories below his. He chuckles to himself, and wanders off to his laptop that's been flashing YOU SHOULD BE WRITING for the last three days.

Later, she must have thought he'd gone to bed because when he comes up from his study to the bathroom, he sees Beckett washing her face in a tiny little pair of underwear that make his mouth go dry immediately.

"Uh... Beckett?"

Her elbow sends the bottle of cleanser clattering to the floor, barely concealing her "What the- Castle?" as she turns to face him.

Then she must remember the no pants thing because her cheeks flush red and her eyes do that deer in the headlights widen he finds so amusing (and simultaneously mildly terrifying because it also means she might end up socking him in the jaw out of embarrassment – or worse: shooting him), then steps backwards as though that will shield her extremely naked legs from his view. She bumps the counter as she does so, cursing, then suddenly glares daggers at him. "Are you going to stand here staring at me all night or can I finish doing this in peace?" She subtly pulls her t-shirt down a little further, unwittingly drawing his eyes toward the movement, and her next barked-out word hits him like a slap. "Castle!"

She takes matters into her own hands (literally), and starts shoving him bodily out the door, hands spread across his chest. He is struck by the desire to grab her wrists and mash her body into his, pull her underwear aside and take her right there against the doorframe, her hair spilling across both their shoulders as he kisses her senseless. He won't, of course, in the interests of remaining a gentleman; but he does suddenly notice the heat radiating off her, the dark burn of her eyes, the dig of her fingers in his skin – it doesn't seem like anger but... surely Beckett's not... turned on by this? But before he can exploit this new and exciting piece of information, the door slams shut in his face with Beckett still on the other side, and he's left in the dark corridor of the loft to contemplate it all on his own.

The next morning he peeks through the gap in her door to see her reading in bed, then swings it wide open so he can run in and flop down and rest his head at her hip. Looking at her upside down, he can see her eyeing him with one of her adorably bemused expressions.

"Good morning?" she faces her book down on her lap.

"A woman wakes up in the wee hours, in a bed not her own, and for an unsettling few moments has no idea where she is. Then, as she remembers her surroundings, she realises how cold and alone she feels, and is struck by the intense need to be comforted by another person. So she gets up, padding lightly down the hall clad only in a t-shirt and almost scandalous underwear—" he sees Beckett's eyes narrow, "—she comes to the only room she can think of going to, where a man waits for her, knowing it was only a matter of time before she gave in." He pauses, grins up at her. "At least, that's how my dream went last night. How did you sleep?"

She slaps him around the head with her book (a paperback, thankfully), and goes back to reading without even a facial expression as a reply.

They hit a snag when he maybe goes to an active crime scene without her and... borrows a couple of things not snapped up as evidence so he can do a bit of sleuthing on his own. When she catches him out the next day and rounds on him, he shrinks. This isn't regular old everyday-irritation Beckett, this is angry Beckett who looks about five seconds away from committing cold-blooded murder. He spares a short moment to wonder if she'd be called to the scene of her own homicide – not that he'd know, of course; he'd be dead.

She doesn't talk to him for the rest of the day. He briefly entertains the thought of asking if tying him up and having her way with him would make her feel better, but when he sees her hand hover over her gun as he approaches her desk (she's not even looking in his direction; he doesn't even know if she's realised it's him, and it's simultaneously the most terrifying and amazing thing he's ever seen) he thinks better of it and simply says goodnight, Beckett before he heads for the elevator.

When she starts going out with Demming, he thinks it could be all over.

Then, when the scalpel-wielding heartthrob of a motorcycle man comes along he almost gives up right then and there, standing next to him in the middle of the precinct. Where does he get off being so tall, anyway?

Then, then a case comes along and switches up the game a bit. And not only because by the end he's in possession of an entire business, either.

See, after midnight strikes everyone starts making their excuses to leave until it's just the two of them who can't seem to find one, still crammed into the corner of a booth at the back of the bar. The lights are dim and warm; scotch-haze still stealing over them as she sits with her legs crossed up under her on the wide seat, his arm shifting against hers every time he breathes.

He pulls at her elbow, lopsided grin heating an ache in her stomach, which surges when he asks may I have this dance?

Her eyes narrow but a smile tugs at her mouth, and she takes his hand, pushes the ache down, and laces her fingers through his as he leads her out to the middle of the bar. His hands rest heavy on the fabric of her shirt, and she finds herself sliding an arm around his shoulder, resting her head on her hand and letting him rock them slowly across the floor. For a moment, she doesn't think about how it's Castle.

"We should go on vacation to the Bahamas." Then, of course, he opens his mouth.

And as used to his random outbursts as Beckett is, this one is slightly more outrageous than usual (which is an achievement considering how often words like CIA and time travel come up in his vocabulary), so she pulls back to look at him properly – noticing too that his hands don't leave her hips. "Castle, I'm curious. Are you going through some kind of mid-life crisis? I mean, in the last three days you've bought a bar and more than one obscenely priced bottle of scotch – now you want a trip to a tropical island?" She brings her hand to his forehead as though checking for a fever will account for his ideas, then drops it back to his shoulder. Instead of the jokey reply she expects from him ("Beckett, are you suggesting my life is already halfway over?"), he just looks at her. It's unnerving.

"If I said I was, would you come to the Bahamas with me?"

She doesn't understand why his expression is completely serious and it must show on her face because he smiles his goofy little smile and pulls her in just a tiny bit closer, drawing that ache to the bottom of her throat.

"I'm not trying to elope with you, Beckett. It's just that recently I've started to re-evaluate, I suppose, certain aspects of my life."

She tilts her head, mirrors his smile. "Are you... okay?"

He nods, presses them flush, and puts his mouth close to her ear. She listens to him breathe. His nose brushes her cheekbone, and when he speaks she shivers at the low, low rasp of his voice.

"There's something I meant to tell you when I got back from the Hamptons but I didn't."

She swallows. "And what was that?"

"I missed you."

Her grip tightens, then releases, and she chooses to avoid his implication. "You did say that, actually-"

"Seriously, Beckett." Before she can get out of it, his arms slide her into a proper hug, and his chin rests on her shoulder, breath huffing into her hair. She frowns, confused. She's not sure how many drinks he's had, but surely it's not enough for him to be pulling her into impromptu hugs where she oddly (and concernedly) feels more like his mother than anything else. When he fairly near buries his face in her neck she gives up, and trails her fingers up and down in his hair.

"You sure you're okay, Castle?"

He addresses his next statement to her shoulder. "Two people are dancing in a bar."

She's known him too long for this to end in a punchline, but for reasons she still hasn't figured out, she lets him continue.

"The man considers what he's about to say, and has been since they stood up. But he gathers up all of his admittedly mostly liquid courage and says 'I think more of you than you'd like to admit' and the woman looks at him for a moment, slightly stunned. Then, while she doesn't want to agree, she finds herself saying 'I know'. Then, he does this." Picking up the hand resting on his arm, he kisses her palm, and this more than any of his other dirty or inappropriate or just plain ridiculous theories has her heart rabbit-thumping against her ribs.

"We're seeing other people."

His smile is traced with sadness. "I know."

It's over a month before either of them mention that night (she never stops by the bar, and his heart is dangerously close to a dead weight).

A few days after he breaks up with Gina, they stop by the elevator, and he feels her eyes on him as he presses the button, waits for the ding.

He doesn't look at her. "Something on your mind, Detective?"

Her voice is quiet. "Two people are standing at an elevator."

Castle starts, and looks to the cop standing next to him, her eyes trained on his face, her eyebrows knitted together as though keeping his gaze is going to take all her strength. She bites her lip.

"The woman has been thinking for the last forty-eight hours how it's the second time in a month that she's flashed back to a moment at the beginning of the summer she'd really like to forget."

His voice feels gravelly in his throat. "And what moment is that?"

She shakes her head. "It doesn't matter." She takes a step closer. "I'm sorry about Gina."

If he were going to weave a theory about their current situation, this would be the part where he just kisses her because can't think of a word to say; his entire being focused solely on Beckett and the question she seems to be asking without asking anything at all.

But he never gets a chance to practice it, because she kisses him first.