A/N: I've never written anything M rated before, but what Ellie wants, Ellie gets. So, uh, yeah. This is. . .a little awkward.

For Ellie. Co-latha breith sona. Tha gaol agam ort.

amantes sunt amentes

(lovers are lunatics)

"So. . .this is fun."

Castle snakes his hand into the gap between his seat and the door and feels for the handle, turning it a little so he can recline. Beckett huffs at him, rolling her eyes as he lies back and arranges his bones in some approximation of comfort.

Whatever. It feels like they might be here a while, and there's nothing that says he has to sit through the stakeout in discomfort.

Not that Beckett's car allows for much of anything else.

"I did say you didn't have to come."

Ohhh, so much he could do with that. But no. No.

Things are so tentative between them lately. Ever since the Ryans' wedding, when she let him hold her close, put his hands on her to the lilting tones of the live band. He keeps waking in the night with the phantom heat of her body pressed against him, his throat dry and aching.

He won't take the bait. Instead, he turns to look at her, a lift to one of his brows that draws her smile out, flushed and gorgeous. "No, Beckett. As your partner, it's my duty to keep you company during stakeouts. Especially since the guys aren't here."

She thought it would be less conspicuous to only have one car parked out front of their suspect's house. Same reason they're facing the wrong way, Beckett keeping watch in the rear-view mirror instead.

Doesn't matter to him. They both know he'll spend most of their night watching her instead.

"Yeah, but I'm sure you have other places to be." She says, carefully not looking at him. He sees the thread of tension in her jaw, the shuttered fall of her eyelashes, and it's a battle not to reach out to her.

"There's nowhere I'd rather be than with you, Kate."

When she turns to face him again, it's a full body thing that speaks of permanence. Her eyes still flit to the rear-view every so often, but they're soft on his. Dark and intense like the inside of midnight. Her mouth parts like she's going to speak and he waits her out, but there's nothing.

This is his part of their push and pull. Bridging the gaps with his words. Only, right now he doesn't want to push too hard.

The case. Stick to the case. It's safer that way. "What's our plan here?"

"He might try to run. So we just keep watch, and make sure he doesn't."

"And if he does?" Castle can't help but reach out to nudge at her, a slow smile easing onto his face.

Kate sighs, turning away from him again so he gets the slope of her nose, a shard of cheek. "We catch him."

The moonlight paints onto her cheekbones, liquid gossamer light that falls fluid over her so she seems ethereal, pale and stunning, and he just can't help himself.

"You are so beautiful."

It's out before he can stop it, and he sees the quiet catch of her breath in her chest, a forever moment of carefully pieced-together nothing. "Castle-"

"Sorry. I'm sorry, Kate. I'll shut up." He rests his palms against his thighs, curling his fingers into the flesh there and feeling the hard stretch of muscle where he strains not to move, lean in to her.

Something about this night, the amniotic quiet of the car and the molten line of Kate's body stretched out in the seat, shatters his control and leaves it scattered through his gut, sharp when he moves.

"You shouldn't have to censor yourself." She murmurs, her voice cold with guilt.

He doesn't want her to feel guilty. Not for this.

"Oh what, so now you want to hear all my crazy theories?" He manages a laugh, manages not to say instead you're not ready, Kate. Even though they're both thinking it.

"I thought I already do hear all your theories."

Oh, to hell with shutting up. She needs to know that he's okay with waiting. That it's fine by him, as long as he's waiting for something.

"On the swings that day after you were shot, you said you can't have the right kind of relationship until that wall inside you comes down. I'm really looking forward to that day."

He's quiet, quiet enough that he hears the soft whimper she lets out in spite of herself. Damn it. He doesn't mean to hurt her, drown her in the tide of his stupid heart.

"I want to be ready so badly, Castle." She grits out, her whole body an oncoming storm. "I just don't know how."

"I was thinking. What if we're doing this all wrong? You're trying to knock down a wall by yourself. What if you let me help?"

Kate's teeth sink into her lip, a whitewash spanning from the point of contact right out to the corners of her mouth. "Help how?"

He reaches across the center console and takes her hand in his. No hesitation, no pretence, just the kiss of their palms and the knot of their tangled knuckles. "Well, I could hold your hand."

"Not at the precinct." She gives him a look past the curling ink of her hair but her thumb sweeps back and forth over his pulse, her cheeks flushed with warmth.

"Of course not, but other times. At home, in the park, wherever." He grits his teeth to stem the flow of hypotheticals, all the ways he wants to love her.

Kate smiles, a bright arc that he last saw amongst the smouldering rubble of a bank. "Okay. Hold my hand. Any other suggestions?"

"Well, I don't know that you can handle the full force of my destructive capabilities, Detective Beckett." He offers her an out, levity if she needs it.

Your move, Kate.

"Oh come on Castle, I know you have ideas about this." She rolls her eyes at him, and he can't help but wonder.

How far is she willing to let this go tonight? She's pushing herself to be open, he sees that much. And that's terrifying. If he goes too far and ruins all his efforts, four years' worth before this one pivotal night, he'll never forgive himself. But God, he wants her.

So badly. And she's asking.

"Well, I do have-"

"Shit, Castle, he's looking." His eyes snap to the mirror and he can just make out the hazy outline of their suspect framed in the window, a slice of him visible where he's tugged aside the curtains. "We look suspicious."

"Should we leave?" Rick watches her, sees the whir of the possible scenarios flitting past like ticker tape in her eyes.

"No. That's even worse. I'm gonna-" She glances at his pants – Beckett, what –and then she sighs softly and tosses both legs over the center console and climbs into his lap.

"Kate." He grunts, his hands falling to cradle her hips in spite of himself. "What are you doing?"

"We need to look less suspicious and there is only one good reason people sit in parked cars in the middle of the night, Castle." She's sighing at him, he can hear the lace of it in her words, but he thinks there's some of the same hot rush of lust that's tearing through him in it too.

He manages a shaky laugh, his whole brain stripped back to the heat of her in his lap and the press of her knees bracketing his hips and her mouth so close to his. "To watch their suspect?"

"To make out." She snaps, and then she sinks down harder onto his lap and oh God he just has no control when it comes to her and she must feel just how much this is doing it for him. There's no possible way she doesn't.

"Shit, Kate. Shit. Are you sure about this?"

She plants her hands at his shoulders and rocks her hips into his and his eyes slam closed, drowning in a burning wave of her scent and how soft her skin is where his fingers snake underneath her shirt. "If we don't do something he's going to figure out that we're watching him. We don't have a choice."

"I don't want to write this off as undercover. Not again." He murmur

s into her neck, his tongue already out to taste the musk of her.

"It's not undercover. It's not."

"Are you sure?" He rakes his teeth along the cliff face of her jaw and she throws her head back on a moan, clutching harder at him.

"Kiss me."

And he does.

It's wet and a little sloppy and absolutely perfect, the first (third) time his mouth meets hers. Kate can't help but whimper into it, sucking his bottom lip into her mouth until he groans, pulling back just enough that he can slide his tongue inside.

He clatters over the sentry rows of her teeth, the roof of her mouth, the slick slide of his tongue over hers sending heat curling low in her abdomen. She gasps, every ragged breath tasting like him and the bite of cold outside that still lies tangled in his clothes and hair.

His mouth comes away from hers with a noise that is utterly obscene and he's laughing even as he descends to suck on her pulse. He laves at her, turning the rush of her blood to stuttering arrhythmia - Oh God oh God Castle – and then catching his teeth in the paper-fragile skin so she jolts and her hips grind into his.

The heat of him between her legs makes her cheeks flame and she bows her head, her mouth open and seeking the rasp of his stubble against her tongue.

Somehow, his hands are under her shirt and cresting up, up and then he's cupping her, groaning something into the dip of her clavicle that might be her name. He catches her nipples between his fingertips and pinches, the rasp of lace against her enough that she cries out.

Oh, God. He's not playing around here. They're really doing this, in the damn passenger seat of her patrol car.

Okay. Okay. It's fine.

She does want him. She does. The slippery heat between her legs is proof enough of that. She just doesn't want to ruin this careful understanding they have between them.

Castle must feel the tension rip through her because he stills, pressing his skull back into the headrest to meet her eyes. "Do you want to stop?"

"No." She half-sobs, pressing her forehead to his and finding his mouth again. It's softer now, more tender. "I want you so badly, Castle."

"I don't want you to regret this." He tucks the thick curl of her hair back behind her ear, smoothing his thumb over the rise of her cheekbone.

"I won't. I won't. Please, don't stop."

He smiles into their kiss, his teeth testing the fullness of her lower lip as his hand continues working under her shirt. The clasp of her bra falls open easily and he gets his hand on her bare skin, swallowing her groan.

Kate slides her hands down, clutching at the material of his shirt to tug it free from his pants. She won't risk taking it off of him, not with their suspect - and God knows who else - still watching, but she wants to feel him.

The skin of his waist is like gossamer and he shudders when she brushes the pads of her fingers over it, etching her heart into him.

"God, Kate." He gasps, kissing her harder. He's scattering the touch of his mouth all along the edge of her jaw, her neck, coming back to her lips again and again like a lodestone.

And then he sucks her earlobe into his mouth and she grits her teeth, rocking her hips hard against his.

He laughs, one hand sliding out of her shirt to busy itself at the button of her pants. There's a moment of hesitation, both of them frozen in place, and then he drags her zipper down. Slow.

The vibration tears through her and she keens against the side of his neck, biting at his shoulder as he slides his hand into her pants and cups her brazenly.

"Oh fuck, Kate, you're so wet." He moans into her mouth, and she shifts in his lap, working her hips against his hand.

He's infuriatingly still, and she bites at his mouth in retaliation, hard enough to make him jolt. "So do something about it."

Castle nudges the soaked cotton of her underwear aside, sliding a finger against the heat of her and coming up to circle her clit.

She can't help the ragged, desperate noise that tears out of her throat, or the thrust of her hips against his hand. He dips a finger inside her, shallow and teasing and cruel.

God, she hopes their suspect isn't watching any more. She can't manage to keep her eyes open long enough to check, not with the slow thrust of Castle's hand stealing every breath.

"More." She chokes out, and he smiles into her mouth, pressing another finger into the slick warmth of her.

And then he's curling and thrusting and his thumb against-

Oh God Castle please oh God yes Castle yes yes yes

She shudders in his arms, her mouth open against the fabric of his shirt as it sweeps through her. And then, boneless against him, she can't help but laugh.

Castle just made her come. Hard.

She kisses him, a taste of drowning to it, and he groans into her mouth. "Kate, Kate, I want to taste you. Oh God, please."

"There's no room for cunnilingus in the passenger seat, Castle." She hums, slender fingers tripping down to work at the button of his pants.

He growls, thrusting his hips up into her, and this is the first time she's seen him lose his tightly wound control. Maybe ever. "Oh my God. You are so hot."

She laughs, and then his fingers slide out of her and she whimpers, feeling the loss of him down into her toes, and it's not funny anymore. "Castle, I need you inside me. Now."

"Are you sure? Once we do this there's no going back." He's suddenly serious, the shared heat of their breath condensing in the sliver of space between them.

"I think we've already gone so far that there's no going back, don't you?"

He doesn't laugh. Instead, he cups her cheek in his palm and dusts his mouth over hers. "I don't want to push you."

"You're not. I want this just as badly as you."

"Okay. Okay, then you need to know." He swallows, his eyes shuttering closed, and when they reopen she sees it there before he even says it. "You need to know that I don't want this to be a one-time thing, something we never talk about again. Because. . .I'm in love with you. And I won't ever stop wanting you."

"Castle." Kate whispers, her forehead crashing in to meet his again. "I don't want a one-time thing either. I want to do this."

"Kate." He breathes her name into her mouth, at once kissing her and tugging her pants down her hips, his thumbs caught in the band of her ruined underwear. He gets the clothes down to her knees and then she has to help him, struggling to free herself of them.

She tosses them into the backseat and sits back, lets him look at her. His face is open with awe as he watches her hands work to tug his own pants down, lifting his hips to let her free him. She cups him in her palm, squeezing gently and sliding just enough to make him cry out.

"Something you want, Castle?" She grins, a little twist on the end of her glide that makes his whole body rock into hers.

He arches his neck to kiss her, his hips shifting impatiently between her thighs. "I want to feel you. Please, Kate."

Kate drops her hand and shifts forward, grinding her hips into his. He's thick and hard and straining between her legs and when she rocks her hips he moans. She does it again, and again and again, until they're both soaked in her wetness and Castle is panting into her neck.

"Now. Kate, now."

She wraps her fingers around him again and sits forward onto her knees, guiding him so the very tip of his erection presses against her. He holds his breath for a moment, and then he opens his mouth to beg and she sinks down, taking him inside.

Oh, God.

Neither of them moves for a beautiful, blissful moment, Kate's internal muscles straining around him.

It's been. . .it's been a while, and he's-

He's not small.

"You okay?" He gets out, one hand braced against the car window. The heat of them has made the windows steam up so he leaves a handprint against the glass, barely enough breath spared to laugh at the cliché of it all.

Kate rises and sinks back down onto him, her palms braced against his shoulders. "I'm good. Now move, damn it Castle."

"Right." He grins into the crease of her neck, sucking on her pulse again as his hips thrust hard into hers.

Shit. He feels so, so good. Kate throws her head back and clutches at his shoulders, her eyes closed as heat coils low down, tight and urgent.

His thumb slips down to circle her clit again, his fingertips brushing up against where they're joined. He shoves her shirt up, biting down hard on her nipple so she bucks into him. Oh God, she's going to-

Not yet not yet please not yet.

"Wait, wait, stop." She manages to grit out, waiting for his hips to still under hers before sliding her tongue into his mouth.

He moans, the seismic tremor of it tearing through her as his hips twitch between her thighs. "Kate?"

She kisses him again, bruising and desperate, and then "Now. Hard." into his mouth.

Castle groans, driving his hips into hers with abandon and gasping out a long, guttural string that might piece together into her name if she had the energy to spare deciphering it.

He thrusts and thrusts and thrusts and the fire rips through her, burning down into her toes and coiling in her lungs and so close so-

Oh yes oh fuck Castle oh God

She clenches around him, hard, her hips still desperately working as her head drops to his shoulder and he spills inside her in a white hot rush of bliss.

His arms band around her back, keeping her close against him as his chest heaves and he presses a line of kisses to her jaw, the melted togetherness of their skins making her want more. Him, shirt off and stretched out over the length of her liquid body.

"That was incredible." He sighs, kissing her with so much tenderness that she can't help but cup his cheek, keep him in place.

"Mm." Kate hums her agreement, moving to shift off of his lap and clean herself up.

Castle clutches at her, fingers still slick with the both of them and sliding under her shirt to curl at her waist. "Stay."

"Castle. We still have another two hours until the boys come and take over from us. I'd kinda like to put my pants back on."

He nods, the edge of his jaw a cliff face as he grits his teeth and drops his hands from her. "Right."

"Hey." Kate leans in to kiss him again, lacing her arms around his neck and smiling into it. "This isn't me regretting anything, okay? I just don't want to get cold."

"Okay. Yeah." He seems lighter, but there's still so much doubt in his eyes. He really thinks she can go back to pretending she doesn't feel anything for him.

Oh, Castle. What has she done to him?

Kate climbs back into her own seat, reaching into the back for her clothes and tugging them on awkwardly, feeling the burn of his stare. When she's dressed, she glances back to him and sees he's tugged his own pants back up, retucked his button down.

She doesn't let herself think about it. She just swings her legs back over the center console and straddles his thighs again, sitting further back this time. His knees press against her ass and even that contact has her shuddering.

His mouth opens on a breath of surprise and Kate smiles, leans in to kiss him. Just the press of her mouth to his, again and again until he's grinning and it's falling apart around the clash of their teeth. "Castle. We still have to watch the suspect for a couple hours. But after. . .come home with me."

Her eyelids slip closed as he tenses under her, and then he's beaming again, coming back to kiss her. "Yeah, okay. Home. I'll come home with you, Kate."

The second time he slides inside her, they're both gloriously naked and buoyant in the rippling lake of her sheets.

And after, she doesn't have to move, doesn't have to peel her body away from his. Instead, she lets herself fall asleep to the quiet tattoo of his heart beneath her ear.

A/N: Happy birthday, Ell-Bell. I hope this is something close to what you wanted.