A/N: Inspired by the Kate Beckett kickboxing scene in 8x03, PhDead. Completely AU.

Her knuckles ache, her muscles burn, and her eyes sting from the sweat, but her determination to improve is stronger than the pains lacing through her body and bones.

"Need a partner?"

Beckett rolls her eyes and lands another high kick to the punching bag dangling in front of her, stumbling a little when the bag swings and her leg threatens to go with it.

"I could at least steady the bag for you, Beckett," Officer Castle quips, strolling towards the mat she's occupying. "Or we could do some one on one training if you'd prefer."

She hates her training officer. Of course he knows her tragic backstory, anyone with a file on her does, and while she appreciates the lack of acknowledgment to the true reason for enrolling in the Police Academy and graduating into the final leg of her training to become a homicide detective, she still doesn't like him.

He's smug and charming and too good to be a cop, too happy and carefree, and yeah, maybe it's a little ridiculous to despise him for having a pleasant attitude, but after embracing the hurt, the anger, and the numbness for so long, she doesn't exactly do well around the joyful.

"Don't you have something better to do?" she mutters, flexing her fingers, wincing at the splinters of soreness rippling from her knuckles.

"I'm your training officer," he reminds her, as if she's managed to forget. "It's kind of my job to train you, so..."

"Then train me," Beckett spats, her hands falling to her sides in exasperation, but Rick Castle merely smirks at her and comes closer.

"Let's start with your form," Castle begins standing opposite her with his hands raised in a block. "Try to sidekick me."


Kate takes a breath and balances on her left foot, grits her teeth and extends her right leg into a swift kick that should land a blow to his throat, but Castle catches her ankle, spins her away, and sends her crashing to her knees.

"Too low and too fast, left you unsteady. I shouldn't have been able to knock you down like that."

Beckett growls under her breath but gets back to her feet, positions herself for another try.

"Tighten your core," Castle instructs, preparing for her next strike. "Focus on where you want the kick to land."

She grunts with the next kick, grazes his sternum before Castle snags her ankle again, but he doesn't throw her away this time, holding her there.

"Find your balance."

"Let go of my leg," she huffs, on the verge of tipping over, but Castle doesn't release her and she shoots him her worst glare as she steadies herself, ensuring she'll no longer sway on her single limb.

Officer Castle lifts her leg higher, so the sole of her foot is positioned at the base of his throat. The stretch of her muscles burns with the extension, but it's good. So is the heat of his skin against hers.

"Feel the difference?" he asks, quirking his brow at her and lowering her leg, releasing her.

"Yeah," she breathes, reeling her leg back in and bouncing on the balls of her feet to eradicate the sudden tension zipping through her veins. "Let's go again."

Kate sweeps her leg between his, tangles their feet and sends him down on his back, but Castle manages to hook his heel in the back of her knee, bringing her down on top of him.

A gasp of surprise rushes past her lips as they collapse together, his laughter rumbling beneath her chest, hearty and infectious. They've been sparring for the last hour and the ache in her body has gone numb with the promise to return in the morning in the form of stiff muscles and aching joints, but she doesn't care. She finally took down her training officer after being the one on her back for the last sixty minutes.

"You're a quick learner," Castle pants and she notices he's grinning when she lifts her head from where it landed on his shoulder through their fall.

"Well, you aren't the worst teacher," she muses, using the hands on either side of his head to elevate herself into a hover above him.

Her hair is slipping free from the ponytail she managed to rope it into, the stray strands falling around her face, clinging to the damp, heated skin, and something about her above him, practically on top of him, with her hair threatening to unfurl in a curtain around them has his pupils dilating, the bright cerulean of his eyes darkening to a fierce cobalt that has an entirely different kind of burn spilling through her body.

But before she can even consider doing anything about it, Castle is curling his arm under one of her knees, flipping them over so quickly that the breath is knocked right out of her.

"Never let your guard down, Officer Beckett," he murmurs, staring down at her with one of his eyebrows arched and half of his body draped atop hers.

"Noted," she grits out, resisting the ridiculous urge to use her unrestrained leg to twine with the other around his torso and hold him still while her body arcs into the enticing wall of his.

"I'm going to hit the showers, I'll see you bright and early tomorrow?" Castle inquires, untangling his arm from her leg, rising on his knees and scraping a hand through his sweat dampened hair.

"Tomorrow," she confirms, remaining flat on her back on the mat with her chest heaving and her eyes closed, listening to him walk away and waiting until the shower has cut on to force herself up and to her locker.

She still hates him, she does, this is just... physical attraction. Totally normal, completely manageable, and definitely not an obstacle that will get in the way of her true goal for being here.

Justice for Johanna Beckett. Nothing else matters.

She has room for nothing else.

Training in the precinct gym with Castle is not the only form of teaching she receives from him, but it's unquestionably become her favorite. Sure, working with him and her fellow trainees on the firing range or roleplaying situations they could face out in the field is always informative, but this - the physical intensity, the burn that reverberates from her lungs to the rest of her body, the contact with her training officer - provides her with a form of tangible knowledge and a release she's grown to crave.

They're nearing the end of their daily session today and she has his head in a lock between her thighs.

"Okay, now use the momentum of the roll to flip me."

Kate exhales before following his instruction, tightening her inner thigh muscles and rotating them the necessary 180 degrees until she is no longer on her back and his is flat on the mat. Castle attempts to reverse the move, hook a leg around her calf to bring her tumbling forward, but Kate plants her knees firmly in the mat, pins him beneath her in a maneuver that has him groaning.

"Submit," she growls, jerking his arm in a twist he taught her last week, squeezing the knees bracketing his ribcage.

Rick slaps his free hand to the mat and Kate releases him, lets him flop back down, panting and breathless.

"Has the student become the teacher?" she teases, still attempting to catch her own breath, dropping forward to rest her hands on the worn surface of the mat beside his head, hovering above him once again. It's not exactly an odd position considering their activity, but Castle always notices the shift, that her stake of dominance holds a hint of something more.

"You're younger than me, the extra stamina gives you an advantage," he counters, his leg twitching, and Kate sinks down, drapes her body over his before he can even think about trying to regain the upper hand.

"Sore loser," she hums. "Does this mean going all night is out of the question for you?"

Castle chokes on a laugh, but smothers his smile quickly, leveling her with a reprimanding look instead.

"You'd be surprised what a man of my age can do," he informs her and oh, that sounds like a challenge, one she so badly wants to accept.

"Oh yeah?" Beckett returns, shifting above him, rolling her hips ever so slightly. "Surprise me."

Castle startles beneath her at the movement, as if being snapped from a haze, and immediately begins reaching for her hips, ready to remove her.

"Officer Beckett-"

No, no professional bullshit. Not now.

Kate grinds down, her hips clashing with his, and his hands grip her harder, bruising as he goes still.

"Kate," he breathes, eyes screwing shut. "We can't."

Indignation flares hot in her chest, but it's too late. Her body is already wound taut with tension that has existed from the day they started these private training sessions, from the day she met this infuriating man and felt the devastating attraction burn through her grief, and she can't stop now. She won't.

Especially not if he wants her back.

"Why?" she challenges, lowering her cheek to press to his, the breath of her question washing over his ear.

"You know why."

"A stickler for the rules all of the sudden?" she mumbles, tightening her knees at his waist, circling her hips once more, and feeling the groan building in his chest.

"I'm not a good person to be involved with," he tries to reason with her through gritted teeth, the hands at her waist loosening. "Not in a good place to be involved with anyone either."

"Neither am I, but I'm not asking you to be my boyfriend, Castle," she states, withdrawing just far enough to see his face, to stare down into the murky pools of arousal bleeding from his blown pupils, overtaking his irises.

"Then what is it you're asking for, Kate?" he murmurs, those wary hands gliding from her hips to the small of her back, grazing the skin above the elastic waistband of her pants, left unprotected by the tank top that has ridden up her spine.

"To feel something." Her hips rock hard into him, her lower body grinding with intent to prove her point, to feel the heat simmer in her abdomen. "Just-" Her lips brush his mouth as she speaks and the electricity crackles where she can taste his breath. "Make me feel something."

Castle surges upwards, taking her mouth and fisting one hand in her hair, sending the sparks of electricity bursting.

She stops trying to control the quivering need in her hips, allows them to resume their sensuous dance against his that he finally begins to participate in, lifting to meet the rock of her body with his own as his mouth devastates her. Kate moans at the slick of his tongue to the seam of her mouth, the plunge inside the moment her lips part for him, and glides one of her hands beneath his damp t-shirt, relishing in the sweat of his overheated skin scalding her palms and feeling the muscles of his abdomen ripple under her fingertips.

"We can't do this," he gasps, tearing his mouth away from hers, using the fingers ensnared in her dilapidated ponytail to stop her from chasing.

"Don't you dare-"

"Here," he clarifies, nipping at her bottom lip to emphasize that he's not trying to stop her, this. "Showers?" he asks, as if for permission, as if she isn't the one who pinned him to the floor and trapped him with her weight, with her want and her need.

Though, he could have easily overpowered her. If he had wanted to.

"Yes," she breathes, nodding eagerly and prepared to stagger to her feet, drag him to the adjoining room of showers, but Castle curves his large hand at her ass, securing her body firmly against his as he rises to his feet.

Kate locks her arms around his neck, her legs around his waist, and scrapes her teeth to his jaw while he strides to the locker room. They end up against the wall beside the sinks first, her back to the cold tile as Castle pins her hands above her head with only one of his.

"Fuck, yes, just like that," she hisses, dropping her head back against the wall as he sucks hard on her pulse, undulating his lower body rough but perfectly into hers, imitation of what she could have if he was naked already. "Clothes - get them off."

Castle's hands snag in her tank top first, stripping her of the fabric and admiring the hard lines and defined planes of her upper body, palming the mounds of her breasts through the black fabric of her sports bra.

She stabs her bottom lip with her teeth to bite back the whine lodged in her throat and instead focuses on using her newly freed hands to jerk and tug his shirt upwards, working at his pants the second he reaches for the hem of the shirt himself.

By no means will they be going slow, but Kate does spare a moment to appreciate the muscles of his chest, his arms, his back, all taut and rigid beneath her wandering hands, contracting at the touch of her fingers. He radiates strength and security, and she hates it, hates that she loves it, but he makes her feel safe, protected.

Her abdominals jump at the graze of his fingertips on an exploration of their own and her back arches harshly when he slips them beneath the damp edge of her sports bra, peeling the exercise gear from her skin, over her head to drop it into the growing pile at their feet.

"Rick," she breathes his first name for a change, knotting her fingers in his hair as his mouth dusts along the slope of her breast, the heat of his breath coating her nipple before his lips close around the taut peak, his tongue adding gasoline to the fire already consuming her system.

Goosebumps erupt along her flesh as he travels across her chest with his mouth, showing her opposite breast the same attention while simultaneously pulling her from the wall. Her body curls around him, around the work of his mouth, clinging as he maneuvers his way into the nearest shower stall.

"Pants," she croaks, snagging the fabric of his workout pants in her toes and tugging hard enough to drag the elastic waistband past one hipbone.

Castle helps her feet find the tile floor, holding to her outer things through the descent of her legs from around his waist. She wobbles for a second, but he doesn't smirk, too busy hooking his thumbs in the waistband of her bottoms and sliding them down the long lines of her legs, pausing between them.

Her shoulder smacks into the cold water nozzle when he places his mouth on her, gliding his tongue through the soaking slit of her folds.

"Cas-tle," she pants, her knees growing embarrassingly weak, trembling at the nudge of his nose to her clit. "Want you. Now."

Rick makes one last swipe through the wetness between her legs, devotes a moment to suckle at the swollen bud of her clit and pull that whine she had been withholding from her throat.

She breathes a sigh of relief when he steps back to remove the last of his own clothing, kicking both pairs of pants from the shower stall and reaching for the shower nozzles without looking, his eyes piercing and solely on her. The water comes spilling through the shower head, sharp and cold, but barely noticeable when she has Richard Castle pressing her into the slick tile wall, clearing her head with his tongue in her mouth better than any cold shower ever could.

"Lift me up again," she demands, her body already slipping and sliding against his, the friction delicious but lacking the leverage she wants.

Castle huffs, but complies. Partially.

"You're bossy," he comments, hoisting one of her legs up to curl at his hip, but not the other.

Kate blinks through the warming spray of water to raise an eyebrow in challenge, but the tip of his cock is slicking through her folds, teasing at her entrance, making her writhe and buck forward for him.

He doesn't torture her any longer, pushing inside in one smooth thrust and smothering the moan she can't control with the seal of his mouth to her lips. Beckett coils her arms around his neck, her nails piercing the skin stretched across his trapezius muscle, and hums into his mouth in appreciation when he hauls her other leg up to twine around his waist, deepening the angle.

Her training officer has slammed into her, tangled limbs with her, and pinned her down with his body multiple times before, but never like this, never this good.

"Harder," she begs, not even ashamed of the need in her voice. Because she needs this, needs him and this glorious wash of heat and overwhelming sensation that only he has been able to elicit and send coursing through her bloodstream. And something tells her that maybe he needs her too.

Castle withdraws almost completely from her, plunges back in, and the stars begin to burst in the corners of her eyes.

"Close?" he gets out against the hollow of her throat, licking at the water gathering there, and she gasps out an affirmative response, shuddering when one of the hands supporting her thighs rises to steal one of hers from his back, pressing it into the wall above their heads, filling the spaces of her fingers with his.

The starbursts burn brighter behind the lids of her eyes, exploding in blinding colors as her inner muscles seize around him, coaxing his release to coincide with hers and sending them both shattering.

She thinks if the muscles of his legs were not so well disciplined, they would go collapsing to the floor, but Castle holds their combined weight, allowing only their interlocked fingers to slip from their place against the wall, falling to rest near her hip.

Kate peels her eyes open, a great effort with the water clinging to her lashes and the final vestiges of her orgasm still swimming through her senses, and tilts her head back into the wall, gulping down air and the mist of the shower. A harsh shiver wracks her body at the absentminded touch of Castle's lips trailing along the cords of her neck, up to her chin.

Mm, it's nice, but intimate, too personal, and that isn't what they agreed upon.

Kate clears her throat and wiggles her fingers free from his, holds to his shoulders to keep from slipping. Though the wet slide of friction is so good, she wouldn't mind it again, but no. No, enough for now.

"We should-"

"Oh, yeah," Castle murmurs, gripping her waist to balance her as she unhooks her ankles and retracts her legs, biting her lip when he slips from her. "This doesn't change anything, Beckett. We-"

"It changes nothing," she repeats with a firm head nod, locking eyes with him through the spray of the shower that has done little to make her feel clean. "It's just... a bonus, to my training."

"Kate," he huffs, wiping at his eyes. "Don't put it like that. It makes me feel like I'm taking advantage of you or-"

"You should know by now that I never do anything that I don't want to do," Beckett states, planting both hands on his cheeks and holding his gaze. "You may be my superior here, but I'm an adult, Rick. We're two consenting adults and by no means are you taking advantage of me. Besides, no one has to know."

"No one can know," he reiterates, using the broad wall of his back to shield her from the pelting rain of hot water. "I could lose my job and you could be suspended."

"Then keep it like this," she shrugs. "Just us."

"It's risky-"

"Yeah, but we'll be careful," Beckett assures him, ignoring the tiny, nagging voice in her head that sounds suspiciously like the M.E in training she recently met during their tour of the morgue a couple of weeks ago telling her this is a bad idea. "We'll go somewhere else next time. Out of the precinct."

Castle purses his lips for a moment in contemplation, but she knows she has him, doesn't have to wait long for him to confirm it.

"Your place or mine?"

"Yours," she mumbles, drifting forward to graze her knuckles over his navel, traipsing down, down, down until her fingers are curling around his hardening length, caressing until his hips jerk forward, pin her to the wall once more. "But for now, let me take advantage of you one more time."