A/N: I didn't think I'd manage to get this out, and I haven't edited properly so I hope it doesn't have too many embarrassing typos. :/ I'm also behind on my thank you replies - will catch up this weekend but thanks to everyone who reviewed the last chapter! In case you missed it because of FFnet being down last Friday, it's the one at the precinct gym ;)

Warning: Rated M.

She finds him behind his desk, his good leg propped up on a chair so it's even with his cast. He types away by the light of one low lamp, wearing nothing but his boxers and balancing his laptop on his bare thighs.

Her hair's still damp from the shower, so she sinks into the chair in the corner to finish combing it out, but she keeps her eyes steady on her partner. Her brilliantly devious birthday surprise had broken the dam of Castle's boredom, and the words for his next Nikki Heat came flooding after.

She's surprised at how little she minds waiting, especially in a relationship that's still pretty new. She could just watch him write for hours. Besides, he'd thanked her very thoroughly the night before, and saw her off to work limber and relaxed and ready to haul in a killer. He'd sent her a text an hour before her shift ended to warn her that he'd been writing for hours and might not be able to stop when she got home.

So she settles in to wait patiently. Normally, she'd be in the soft, long t-shirt she keeps next to his stack of boxers, ready for bed, but instead she's put on a black silk shirt with silver snaps instead of buttons. And underneath, a bra - the most ridiculous bra on the planet - one she bought years ago but has never worn because it makes her look almost comically busty.

Twenty minutes later, Castle finally closes his laptop, and Beckett blinks open eyes that had drifted closed. She shakes off the mist of a very satisfying daydream and rises. Stalks over to his desk. In her hand is his latest Nikki Heat. She stares him down, and he gulps.

"Uh, am I in trouble for something I wrote?" he asks.

Beckett sets the book down gently on his desk and slides it towards him. He drags his gaze over her, over the buttons that are straining, into the shadows beyond, and then his eyes land on the book. The cover isn't flat; it's ramped up like there's something just inside the front. Castle opens the book and recognition dawns in his eyes.

It's a black sharpie.

He gets it. Oh, God does he get it.

Beckett quirks an eyebrow at him, challenging.

"It's so wonderful to meet you, Mr. Castle. Your assistant said I could just come in? I hope that's okay?"

"Of course. Where would you like it?" he asks, in the lowest, sexiest growl he's ever uttered those words in.

Beckett's long fingers dance up to the top button of her shirt. With a pop, she undoes the highest one, revealing just a curve of the red lace underneath.

"You can just sign here." she purrs.

"Always a pleasure for a fan, especially one as beautiful as you, Miss…?"

"Kate. You can call me Kate."

"Kate." He tests the name on his tongue like it's brand new. He's so good at this role-playing stuff. "Unfortunately, Kate, I had a tussle with a pair of skis last week. You'll have to come around to this side of the desk so I can reach." He tugs off the cap of the sharpie with a snap.

She catwalks around the desk, and he hears her heels before he sees them. They're red, to match the scarlet edge of the bra, and the underwear he's yet to see. He stares at them for a second, then lets his gaze trail up her long, golden legs, over honey-warm thighs that stem up high, and he chokes on his own tongue when he sees the stupid-small skirt she's wearing. Has she owned that all along? Since he met her? Has it been in his closet? In any case, it's love at first sight.

This isn't really the sort of thing he's ever fantasized about - having a tryst with a scantily clad fan - but he's more than happy to play this game with this particular fan. Not that he doesn't appreciate the chest signings on a purely male ego level. He's Rick Castle, after all; never one to turn down attention. When his first publisher suggested they market to a different demographic than he'd originally had in mind, he'd been pleased at the inundation of gorgeous women. But no one who'd asked to have skin signed also had the presence to draw out his true self. Until now.

And if someone had ever asked him to wager on whether or not Detective Kate Beckett would one day ask him to sign her body, he'd be living in a cardboard box.

Castle angles his wheelchair and levers his good leg to the floor. He gives his knee a little pat. Beckett's laughing eye-roll almost breaks through the sultry veneer of her character. She's so adorably bad at acting. It's a refreshing change from his previous wives. Not that she's his wife. No - not yet - but this is by far the strongest, deepest relationship he's ever been in, and marriage feels inevitable, somewhere in some rich, true part of himself that he doesn't even know exists yet. And damn, the sex is phenomenal - was phenomenal even before they started this Fantasy Fridays thing - and now every time he gets to be inside her it almost obliterates him with pleasure.

She perches on his lap, warm and heavy, and the the pinprick lance of pain down his leg hardly registers. He can only feel her. Her tiny skirt must be riding up high, because one side of his lap is warmer than the other. He's very glad he's only wearing boxers. Where their bare thighs meet, the skin sticks instead of slides, and Castle holds his breath as she wiggles her ass to get her position just right.

Kate's fingers flutter at the edge of her blouse, then she's sliding the fabric down and arching her back to give him the best access to her cleavage. Castle's signed his share of chests, busty, flat, a rainbow of colors, textures, some with less give, some with more. None have been as perfect as the one before him now. He brings the marker up to her skin, can't help but look up at her face, finds her biting her lip and staring at his hand.

It doesn't take long to sign her, leave the scrawl of his name marked on her. The ink glides on smooth and cool, loops and flourishes and all the panache of the public persona he's spent decades creating.

"So, Mr. Castle- "

"Rick."

"Mmm, Rick. Is this where you wrote Heat Wave?" She twists in his lap, runs a finger along the edge of the desk. "Is this where you wrote the infamous tequila scene?"

"As a matter of fact it is."

She traces the edge of his computer with a reverent fingertip. "I've never had a … private signing before." She smiles coyly, and then she's coming for him, a slow lean in until she's close enough to taste. "I hope you don't think it's too crazy to ask for a … little. Tiny. Kiss?"

"Not - no - not crazy."

She rests her lips on his for a split second, soft sweetness breezing over him, before she withdraws.

Lightning quick, he catches her by the neck, fists his hands in her hair as he pulls her back to his mouth, working her lips open, and she's pliant and ready and moving with him like water in one of their trademark make-out sessions. She starts making those noises of hers, little mmms, purring growls, nipping at his lip between swipes of her velvet tongue, and he's completely gone. He likes to think he has some control over himself, but when he's with her, when she anticipates his every need, his every motion, tempo, rhythm, and matches him perfectly, he can only roll in the sensations. He certainly can't think objectively about how to please her. Thank God he seems to get it right even through the haze.

She lifts her weight from his lap, breaks from his lips, and he knows she's either about to plant her knees at his thighs, which is just this side of possible in the wheelchair (they've thoroughly tested) or slide off to lead him to the bed, but he wants to give her more tonight. Especially if her fantasy is him. Rick Castle, the writer. Not the annoying joker who follows her to the precinct, or even the boyfriend who draws her bubble baths and works the kinks out of her shoulders after a tough case. Tonight she wants who she thought he was six years ago.

And the him of six years ago was definitely not wheelchair-bound. It's not like they haven't had sex since his injury - in fact they might be having more than usual because he misses her so damn much when she leaves him all alone at the loft, but the past few weeks have been all about blow jobs (stunning blow jobs) and her on top. He wants to give her what she's really seeking. For her favorite writer to fuck her.

So he uses his considerable upper arm strength to hoist her up off his lap and plonk her down on the edge of her desk. She gasps, half in surprise and half because the surface of the desk is cold on her ass, but before she can move, he grabs her wrist and brandishes the Sharpie once more.

He signs Rick Castle for a second time over the pale blue lines on her wrist, hoping his name follows her pulse up to her heart.

He'd been paying attention when she undid that top snap, and it only takes one tug and a firecracker stream of pops to lay her blouse open wide. He palms her ribcage the way she likes him to, the way that makes her feel delicate and small in his hands. His thumbs drag mercilessly underneath the wire of her bra, and it's almost uncomfortable when he slides his huge hands inside, grabbing the small weight of her breasts and squeezing.

"Mmm. There are the ones I love," he murmurs, and she almost doesn't catch it, or isn't quite sure that she heard right.

"Lay back for me?"

She does.

The cool, wet kiss on her hipbone is all the warning she gets before the whisper sharp sound of ink on skin, and the third Rick Castle graces her body.

And then he's shoving her skirt up and dragging her underwear down her legs. She feels a little awkward, to be honest, laid out on his desk like a feast, but he doesn't give her much time to think about before lifting one leg up, pressing her knee to her chest, and she hears him breathing heavily as he scrawls his name on the underside of her left cheek. It'll be upside-down when she stands up, but she'll remember the predatory gleam in his eye as he marks her a fourth time.

He spreads her legs and signs her again, this time in the crease of her opposite thigh. She feels completely wanton, but she can't help the way she shifts towards him, opens for him like a flower.

And then something's creaking and her eyes fly open to find him struggling to his feet.

"Castle, you're not supposed to be standing without crutches," she hisses.

He doesn't have the wherewithal to argue, or to do much more than shush her gently. His leg throbs in the cast as the blood rushes down, waking up pain receptors and a dull ache that he'll undoubtably regret in the morning, but the sight of her spread out on his desk, wet hair leaving little damp lines in a halo around her, her hand soft and open, palm up over his mousepad… yeah, the sight of her sends an oozing warmth that layers over the pain and drowns most of it out.

Her toes catch the waistband of his boxers and drag them down, one of her weirdly sexy little tricks, and he leaves them at his knees and uses one hand to angle himself down where he'd sprung tightly up against his stomach, and the other to hook behind her knee, taking exquisite aim and taking his sweet time to find her entrance, shove inside, work himself in until he can glide smoothly out and in again.

How have they not had sex on his desk already? They've been dating for months now, and while the office itself has seen plenty of action - on the floor, up against the bookshelves and the window, on the chair in the corner, the desk has escaped their appetites. They'll have to make it a regular in the mix, because it's just the perfect fucking height.

He fucks her slowly, because he can, and because her eyes are milky and unfocused already. All it takes is the press of his thumbs on either side of her clit to make her scream at him for being a tease, and four quick grinding circles to send her over the edge in a tense star-fall of release.

He doesn't want it to be over yet, so he holds on, maintaining an excruciatingly slow pace that matches the throbbing grip and release of her body, and when her eyes finally clear and her color starts to come back, he grabs her ankles, hikes them over his shoulders, and ramps up until the pace is unbelievable, unsustainable. He starts to falter, and his pecs tense, and he knows she can feel him tip over the edge because she's told him how much she loves it - how she can almost feel the sparkles of his ecstasy.

"Sign me again," she pants.

His eyes roll wildly as he tries to process her words, tries to figure out what she means. He reaches for the Sharpie - heaven knows where the cap is now - but she slaps his hand away.

"Sign me again," she repeats, barely audible for all the harsh, heavy breathing from both of them, the swamp of pleasure thick over them.

So he does, filling her once because he can't help himself, then he brings himself out to finish with hot strokes against her belly. He finally stills, shuddering and crumpling over her, smudging the ink at her hip with the evidence of his orgasm.

She holds him tight, their bodies pressed warm together, until she can manage to settle him back into his wheelchair and wheel him to bed.

The next morning, she rushes into the bistro two blocks over to have lunch with Lanie. She'd barely had time to brush her matted mess of hair, much less take a shower, and she's wearing a turtleneck and a scarf just for good measure.

She bites on a grin as she scans the menu, because she wasn't the only one who got signed last night. When Castle fell asleep after round two, she'd marked him with her careful lettering just above his cock: 'Property of #41319.' She glances down at her phone, to see if he's made it out of bed yet, anxious for his delight upon discovery.

Lanie knows something's up, of course, but doesn't ask her about it until Kate forgets and rolls up her sleeve to start eating, revealing the very obvious signature on her wrist, belonging to one Rick Castle, master of the macabre.

A/N: Wow, I never knew how difficult it was to write exclusively smut! How are you guys liking this series so far? Is it too descriptive and naughty? Or not naughty enough? I feel like a total perv sometimes when I write these, and have to work up my courage to post every week!

So I can make sure future chapters are what you want, if you have a second, let me know which one has been your favorite so far: Shower, Car, Workout, or Private Signing. ;)