Mostly my own fault, but egged on by ACertainZest, who is a bad influence.

Chapter 1

"You know," Castle says lazily, shortly after Hallowe'en, "you spied on me."

"You said that. I don't agree."

"Oh, you did. I don't think that was very fair. You knew everything I was thinking and I hadn't a clue about you. That's not equal."

"All's fair in love and war." She smiles sleepily, and stretches languorously. "Which would you like the rest of this evening to be?"

"That sounds like blackmail to me."

"Not at all." She smirks.

Castle pulls her over him and proceeds to prove that love is much nicer than war. She wiggles seductively and encourages him to prove it in a number of ways, all of which are very pleasurably satisfying.

Castle, however, is not entirely satisfied. He feels, quite reasonably, that Beckett had a very unfair advantage over him, however gorgeous and pettable Onyx-Beckett had been and however gorgeous and pettable sexy-human Beckett is right now. He'd told Onyx everything he'd never dared tell Beckett, and, okay, it had all turned out pretty damn well in the end (he strokes over the beautifully naked, snuggly results) but she'd still cheated.

He cuddles her in (after all, who wouldn't cuddle a naked Kate Beckett?) and thinks about that. Cheating is unfair. Snuggling up to him as a very pettable cat, sneakily demanding affection, and then draping bonelessly over him as he petted – now then. Now, then. There's a thing that'll repay research. Because for the last two hours Beckett's been purring and making sexy noises that bear a considerable resemblance to her cat's purring whenever he petted Onyx. Hmmm.

Revenge, as they say, is a dish best served cold, and this revenge will be very, very sweet.

He's not being mean, either. Just like Beckett had been sneaky and spied on him to move them into a relationship so that he's very, very happy, he's going to be sneaky and research some interesting discontinuities in Onyx's behaviour as compared to Beckett's, in order to make her very, very happy.

He smiles sweetly to himself and wriggles them both into a very much better alignment, involving Beckett being rolled over. She reaches up, tugs his head down, and takes his mouth in a fiercely possessive kiss. Castle fights back with equal possessiveness, and adds a wickedly erotic slick of fingers through the blazing heat between her legs, which leaves her mewing and writhing frantically and reaching for his hard length to turn him, he is sure, into just as much of a hot mess as she is.

Sounds good to him.

Beckett takes him delicately and – oh god just like that – evilly to leave him gasping and flexing against her and finally finally finally guides him home where he already knows he fits so utterly perfectly and she moves and oh god yes he touches her just where she needs him to and she cries his name and he rasps out hers and they're one: joined, united, two made one flesh.

"You still spied on me, and cheated," he says later, back to cuddling her as close as he can.

"All in a good cause," she flirts airily. "If I hadn't, you wouldn't be here."

"I'd have got you eventually," Castle says arrogantly. "After all, you wanted to be caught. My Beckett-cat."

"Conceited much? I caught you. You were moping at home wishing and not doing anything about it. If I hadn't sneaked" –

"Oh, so you admit sneaking?"

"Never denied it – up to you we'd have continued dancing round each other for years."

"Still not fair."

"It worked. The ends justify the means."

Do they indeed? Mmmm. That's good to know.

Castle begins his research with some careful thinking the next day, since there are no interesting corpses and he can hardly carry out this research in the bullpen. He thinks very carefully over the last few weeks. He discounts the two or three days in which Beckett had been badly hurt. All she did then was sleep and snuggle.

Now. He'd petted her ears the very first time Onyx had shown up, and she'd purred, and then lain soft and lax on his shoulder and purred continuously. She certainly hadn't wanted put down. There had been just the slightest hint of a growl when he'd had to. So she'd certainly liked her ears being petted. She'd liked it each other time, too. Hmmm. He evolves the first stage of a plan. Pet Beckett-Onyx around the ears, and then persuade her to change back for some thoroughly human lovemaking, and along that extremely pleasurable way, discover whether she's already aroused or simply happy. Perfect.

What else had she liked? She'd liked being brushed. Hmmm. Same process. Okay, that's good. His plan is fine, for now. And once he's sure of his conclusions, then he'll work out some delightful revenge.

He bounces happily through the rest of the day until he can bounce off to Beckett's apartment after dinner. Alexis being on a school trip, he needn't be home for three days, if he doesn't want to be.

Beckett is bored, tired and, she admits, somewhat headachy as a consequence of a long day of paperwork. Castle instantly spots an opportunity in the guise of caring concern (well, he does care and he is concerned, so it's not a lie) to start his researches without any difficult dancing round reasons for wanting Beckett to be Onyx.

"Turn into Onyx, then, and I'll pet you," he suggests.

She peers doubtfully at him, slight creases in her brow, her eyes a little dull. "You think?"

"Sure. C'mon. You said that you liked being Onyx if the day was tiring" – she had – "so if I wasn't here I bet you'd have turned into her already" – she nods, carefully – "so just do that and then I'll stroke you till you feel better." Which is certainly one way of putting it. "Some nice snuggling is just what you need." Likewise.

Beckett regards him slightly blearily for a few seconds, and then is abruptly Onyx. She pads delicately on to his lap: a midnight cover of elegant laxity, and settles herself, paws tucked in, tail curled neatly around her body, the end twitching very slightly. The cat sighs quietly and closes her eyes. Her ears relax.

Castle strokes gently along the silky pelt, not taking any – er – research-oriented actions as yet, always moving in the direction the fur lies, spreading his fingers wide to cover her flank to her spine, moving from nape to the very tip of her tail, slowly and soothingly. His beautiful cat-Beckett emits a tiny, satisfied mew, and relaxes further, almost boneless. He keeps petting, and finds that even though he knows it's his badass Beckett, stroking the cat soothes him too. He's not so soothed that he's forgotten his plan, though. Oh no. He'll just delay it a little while he performs the feline equivalent of massaging her temples. There's no hurry. They've got all evening, and all night, and longer.

"I really like this," he says. "You all soft and pettable and contented." Onyx twitches her tail. "I can stroke you as much as we like no matter who's around and no-one would know it's you." Her ears flick. His hands glide over her, and she begins to purr softly.

Castle takes this as a signal to change it up a little, and alters his petting to start at her skull, very lightly and briefly fondling her ears at the start of each stroke. She is shortly so totally relaxed that she might as well be a fur rug, each limb limp and her head between her front paws, eyes shut. Even her tail has stopped twitching. He files this interesting result into the little patch of his brain marked best ways to seduce Beckett, and very gradually alters the pattern of his petting so that it's finally concentrated around her ears and nowhere else. His Onyx-rug is now purring continuously and in a way that closely resembles a thoroughly aroused human-Beckett.

"Better?" he asks. "Wanna come back so I can kiss you? I really wanna kiss you." He drapes Onyx over his chest, which will ensure that when Beckett is back she is also draped over his chest in a perfectly kissable position. He's adjusting to the geometry of shape-shifting girlfriends very rapidly. Then again, he prides himself on both his suavity and his adaptability.

Onyx mews. It isn't exactly agreement.

"C'mon. You've had all this petting and I want some too."

There's another mew, which doesn't sound anything like as snarky as Beckett normally would when he play-whines, and then suddenly he has a lapful of Beckett not a lapful of fur. She kisses him deeply, and wriggles as close as can be. Castle, nothing loth, kisses her back, smooth and slow, and ups the ante by removing her top and not incidentally stroking gently over every inch of skin as he goes; to which Beckett retaliates by opening all of his shirt buttons and likewise stroking the nicely firm muscle underneath, with particular attention to his flat nipples.

Beckett, even changed back, is already totally blissed-out and relaxed from Castle's ear-petting of Onyx. She's happily floating in a sea of contented sensuality, two orgasms to the good already (not that Castle knows that), and very pleased to undertake some mutually erotic petting and then end up in bed. She is already addicted to petting her Castle, in every form, but human will do for now.

Thinking of which…just because she's blissfully relaxed doesn't mean that she can't apply some smoothly sexy actions to Castle. She glides her hand down his torso, not ceasing to kiss him, and arrives at his belt. She takes her time undoing it, so that he's wholly aware of her actions. He comes to full attention, as it were, and smiles lazily.

"Is that a hint?"

"Could be…"

"Mmm. Maybe I should give you a hint or two in return?"

"Mmm."

Beckett nestles into Castle's very receptive form in a wholly enticing fashion, purrs softly and kisses him again, making her views clear. Her belt succumbs to Castle's teasingly soft fingers. His pants button succumbs to hers, as does his zip. So does hers to him. Both of them are unhurried, softly arousing rather than the hot, hard desire that has previously overtaken them; a new way to play. It's still all so very new, and there are so many things they could explore; so much time to explore them in. All of their lives, he hopes and believes.

Beckett's naughty hands have achieved entrance and are tantalising him in a most unfair way. Castle, through a fog of complete lust and sheer desire, lifts her very slightly and slips her pants from her hips to lie disregarded on the floor, giving him free access to a pair of extremely minimal silky bikini panties and all the delights below. He explores, much as she's exploring him, murmuring into her ear in a deep bedroom baritone which he just knows will send sensual shivers down her senses. It does: she squirms even closer to him: skin on skin, and there's barely room for her hands (not that this is stopping her) but he has freedom to tease.

He sneaks under the edge of the delicate fabric and ghosts his fingers over her. Another tiny datum is stowed in the Beckett-box in his brain. She's soaked, and from her noises she's far more sensitised already than he'd expect. He pays very close attention to fine details. Mmmm. Not proof, but certainly suggestive. Mmmm.

He returns to Beckett-petting, since she's certainly petting him, and when they're both all petted-up, smiles sleepily.

"Let's take this elsewhere."

"Mm? I'm happy here. You feel pretty happy, too."

"I'd be even happier in bed. So would you." He slips his finger into her. "Wouldn't you?" he says over her gasp, and gazes hotly at her.

"Might – ooohhhh – be persuaded."

Now might that be a challenge, Beckett? Castle rarely fails to rise to a challenge, especially sexual, and even more especially from Beckett. Now is not going to be an exception. He stops playing with her, stands and collects her up, and rakishly sweeps her into the bedroom.

"Where were we?" he asks lazily, and grins down. "Somewhere like here, wasn't it?" He moves his fingers delicately, and watches her squirm. Her shirt departs, her bra follows, and Castle turns to one of his favourite uses of his mouth: kissing Beckett's beautifully pert breasts. It's especially fun because he can reduce her to mewling incoherence in no time, and mewlingly incoherent Beckett is just plain wonderful. Of course, she likes it too. She likes it a lot. So much, in fact, that she's making those sexy noises that mean she's right on the edge and then she's over.

"My turn," she purrs, and takes him in hand to feather and glide, tantalise and tease and then slither down to take him in mouth and make him groan and growl and rasp her name and come himself.

Later, much later, Castle falls asleep wholly satisfied, next to Beckett, likewise wholly satisfied, and when he wakes finds the ebony form of Onyx tucked into his neck. The obvious thing to do is to wake his gorgeous feline by fondling her ears, and checking his conclusions has, of course, absolutely nothing to do with it. So he does. Onyx opens a green eye, and regards him coolly, which eye, as he continues to fondle, closes again and is accompanied by a continuous purr and pushing of her silky black head into his hands.

"Come up," Castle says, and slathers Onyx over his bare chest, where she lies laxly languorous. He pets less intensively, suspecting her lethargy's source. "Come back, Beckett. Let's play."

She reappears, still wearing the shimmer of sexy nightwear that she'd – eventually – put on and that had – even more eventually – stayed on. He strokes it: irresistibly touchable and even more irresistibly removable. Eventually. For now, he simply strokes smoothly down over her ass, into the back of her lean thighs, as far down as he can reach and then back up to the hot cleft.

"Mmm," he hums. "Your dreams must have been really interesting…"

"Mrrr?" she says back, and presses into his hand.

"Or there's something else going on. Something you've been hiding. I think you've been keeping secrets from me," Castle says annoyingly. "More secrets. You're far too secretive, Detective Beckett. It's very unkind. Anyone would think you didn't trust me."

"Do so," Beckett grumbles.

"But you're keeping secrets," Castle whines. "You shouldn't do that."

"Can if I like."

"I'm going to discover them," Castle points out.

"Go right ahead. Who says I'm keeping any secrets?"

"You kept Onyx a secret," he says, unanswerably. "And you're keeping something else a secret."

Beckett mumbles something unflattering under her breath on the subject of overly-curious writers.

"But right now there's something that's not a secret at all," he says, and draws a line through her overheated, fluid centre. "You want me."

"You want me," she replies, and proves her point on his hard thickness.

"Let's have each other, then," and he pulls her over him and she slides down on to him and this is the best way to wake up ever.

Later, having coffee and breakfast (which Castle, attuned to Beckett's inability to purchase foodstuffs, had brought with him), he affects a great air of I've guessed your secret which is sure to annoy Beckett immensely, and smirks. As he had expected, it takes her less than two minutes to become irritated by it.

"What is it, Castle?" she snips. "You're behaving like you found El Dorado and won't share."

"I found Nikki Heat, which had much the same effect," Castle says annoyingly.

Beckett grouses indeterminately. "I need to go to work," she points out, which is disappointing. "Shift finishes at six, if we don't get a new body dropping."

"More paperwork?"

"Yeah."

She briskly pulls herself together, slips into the coat which Castle is holding for her, favours him with a toe-curling kiss, and smirks. "That'll make sure you don't forget me when you're writing."

"Why, Detective Beckett! That almost sounded like something I would say."

She clacks out. Castle, already in possession of a key so that he can come and go as he pleases (after all, Beckett can come and go at his as she pleases simply because of the cat flap, so fair is fair) locks up behind himself, and ambles home perfectly happily. Observation schedule one: complete. Theory one, proved.

Some time later, he has located the brush which he used to groom Onyx for the photos to show Beckett. That wasn't fair, either. She pretended she had no idea and secretly she knew exactly what Onyx looked like. Humph. He pouts. Then he remembers his plan. Then he thinks cheerfully for a while, and remembers even more cheerfully that Beckett is due some leave and she'd promised that they would go up to her dad's cabin and let the panthers out to play properly.

He can't wait.

Three weeks later, although the weather is unlikely to be clement in the Adirondacks, Castle and Beckett are on their way to the cabin for a few days of vacation. Montgomery had looked at Beckett's accrued leave, managed not to swear, and told her not to reappear until it was in a more reasonable state, since he has no desire to deal with the union rep even if it is all her own fault. Beckett is, naturally, driving. Her excuse this time is that (one) the roads are entirely unsuitable for Castle's expensive cars and (two) she knows the way and he doesn't.

When they get there, Castle leans on the car for a minute or two and simply stares. He's not a country boy, to say the least – itinerant theatre folks don't go out of towns, or cities, and once he was a success he'd stayed in the glitz and glam of Manhattan and signing tours in large metropolises. Therefore, he has never seen an upstate cabin, and it is not at all what he expected. In fact, he'd rather expected a Little House in the Big Woods type building: that being his only point of reference (and only because he'd read them to Alexis).

It's not like that at all. Well, it partly is. It's a log cabin, two storeys. But it has a porch with swing seats, and steps down to a small river. Castle loves it instantly. It calls to a sense of history and place that he'd never known he had. Beckett opens up, and it's homely and cosy inside, with a wood burning stove and old-fashioned décor: upstairs there's a large, timber framed bed which they waste no time in making.

"This is gorgeous," he comments, as they smooth the quilt.

"We used to come every summer." She smiles at the memories. "Dad taught me to fish out there. Since then, I've been here on my own." Her smile turns liquid and open. "This'll be the first time I've had company in years. It's going to be great."

Castle hugs her. He knows what she isn't saying: that she came up here to let her panther out to play, which – until now – she couldn't share with anyone. Even though outside the window the clouds are lowering and rain imminent – or sleet, more likely – they'll light the fire and curl up on the rug and they can be happily feline together. He's looking forward to that.

As ever, thank you to all readers and reviewers. This will be a three shot and is unashamedly PWP fluff.

On a more serious note, however.

As a Londoner, I say that I shall not be bullied by gutless, hatred-fuelled cowards into changing my behaviour; nor shall I be bullied into hatred of others by people who dishonour their claimed religion and their heritage. Not here, not me, not us.