This story is another in the Felis Felix universe. It won't make sense if you haven't read it, I'm afraid, because it assumes the background from Felis Felix, and is an out-take from the time jump in chapter 6. This one is all the fault of ACertainZest. PWP.

"Don't be ridiculous."

"But Beckett," Castle whines, "how am I ever going to be able to watch you shift without having to worry about what you're wearing if we don't test it? I mean, what if you were wearing an underwired bra?"

"I'll worry about my underwear."

"But when you changed back you'd be bra-less." His expression changes. Beckett growls. It has no effect. "I wouldn't mind that," he muses. "Though it would be worse if your shirt or pants didn't change back. You'd have to stay as Onyx." He acquires an entirely unreasonable look of faux worry. "If you had to change to get out of a sticky situation" – he misses her wince and conscious look entirely, which is just as well, because she has changed to get out of sticky situations – "it would be terrible if you couldn't change back. People might think you'd fled the scene. And what if everything individually was okay but all together it added up to too much? What if you lost your gun? Or shield?"

"Never gonna happen. I've been like this since I was nineteen, and I've never had a problem."

"But Be-e-e-cke-e-e-tt," Castle whines some more. "How can you not want to test it?"

"I don't need to test it," Beckett says crossly, and turns into Onyx, turns her back on Castle, flicks her tail equally crossly at him, and stalks off to a small cushion on which he will not fit. He's being pestilent. She doesn't need to test any dumb theories about the percentage of natural fibres versus anything else, because all her clothes (except her gym gear) are wholly natural fibre – wool, silk, cotton, linen – and after more than ten years of being three natured (human, cat, and panther: she thinks it's three even if two are feline – and Castle doesn't know about the third one yet. She's saving that for a suitable moment.) she is pretty sure she knows what the limits are. Even if she's never systematically tested them.

Her tail flicks restlessly. It's just stupid. She knows what to do. Castle doesn't. He can't shift (though she has some plans for solving that, at an appropriate time) so he doesn't get it. Besides which, it would be undignified. Onyx is a dignified cat, and sitting there as a cat with an artificial fibre t-shirt – or worse, bra – draped around her is certainly not dignified. She washes a paw to relieve the embarrassment of that thought, and curls her tail around herself.

Castle flops down beside her, having basely and unfairly dragged over a much larger cushion upon which to flop. She regards him disdainfully. He's being annoying. She is not inclined to pander to his annoyingness. He'll only keep pushing the point, and then she'll be even more cross.

She is now even more cross. Castle has even more basely and unfairly picked her up and put her on his lap. Well, she is not changing back into Beckett. Humph. She's not going to listen to dumb theories about percentages of artificial fibres and the need for scientific testing. No matter how much he strokes her spine and fondles her ears and generally caresses her. She can not be suborned by petting. Absolutely not.

He strokes her all the way along her spine, over and over; and then turns to her ears. It is absolutely not fair that her ears are so sensitive in cat form. It's equivalent to the way he plays with her breasts, in bed. Quite inadvertently, she purrs. She certainly didn't mean to. Castle is being annoying and he doesn't get to pet her and make her purr when she's cross with him. Even if she loves having her ears fondled and her spine stroked in the direction of her fur; even if she adores being snuggled in his lap and petted. He's being entirely unreasonable, just because he's as insatiably curious as Kipling's Elephant's Child. She is not an experiment.

And he should stop suborning her by petting and snuggling. It would serve him right if she turned back into Beckett – but then he'd only kiss her and she's not proof against that either. Castle, she thinks very sulkily, is far too good at all forms of sensual suborning, and worse, knows it, which is undoubtedly why he does it so often.

It's also undoubtedly why he's now plucked her Onyx-form out of his lap and draped her over his chest, which not at all incidentally provides her with a perfect perch to be bathed in his almost-overwhelming scent. In cat form, the combination of cologne and his own aroma is close to irresistible. However, she will resist. There is no way she is going to be the subject of dumb experiments with artificial fibre clothes. She's certainly not spending her money on them, when she'd never wear them.

She takes a deep breath, and is drowned in spicy masculinity. She will not change back and kiss hell out of him. She won't. And she's going to stop purring, too. Absolutely. She attempts a growl. Unfortunately, it still arrives as a purr, and Castle keeps petting in an entirely unreasonably delightful way. She just knows he's smiling. His fingers wriggle all down her spine in her most favourite fashion, too.

"C'mon," he entices. "Stop being Onyx and turn back into Beckett so I can kiss you." She flicks her tail at him. "You're sulking," he says. She is not. She is producing a perfectly rational reaction to a dumb suggestion that she doesn't want to do. Not sulking at all. Besides which, she can't be sulking if she's purring. Everyone knows that.

Castle puts her back down in his lap, and carries on petting. She had been going to stalk off, but she can't resist the nicely judged touch and – ooohhhh, he's brushing her. That is cheating. She's not been able to do anything except melt into a puddle since the very first time he brushed her when he didn't even know she was her. She loves being brushed, and Castle knows it. Every time he's brushed her since he knew about her, they've ended up in bed. She should ask him to brush her hair when she's Beckett, to see if that's as good. It's just as well she can't talk as Onyx, because she would agree to anything as long as he didn't stop. She sprawls out over his legs so that he doesn't miss a single inch of her flanks, and purrs shamelessly.

"That's better," Castle says after a few moments, not ceasing to brush. Beckett-Onyx stretches luxuriously and sensually under the sensations. Mmmmmmmm. Don't stop, Castle. "I like you purring. Now, how about turning back so I can do some other things to make you purr?"

More delightfully bone-melting things? There had been a reason not to turn back, but she's so dissolved in sensation that she can barely remember what it was, and who cares anyway if he's going to do more things to make her purr? An instant later the soft sigh of air announces her return to Beckett, and Castle's eyes burn hot bright blue to see her. His lips land on hers in an instant, pulling her up so that she's plastered against him: his tongue invades, so she fights back with hers. Somehow his hand has sneaked inside her top and – how did he manage that? – unhooked her bra and she's naked from the waist up in his lap and this is entirely not fair. Absolutely delightfully erotic, but not fair. He's far too good at it. At her.

And then he lays her down and nibbles down her chest and plays with her breasts with his mouth – what is this oral fixation he has? He never stops using his mouth: either talking, or he's using his mouth on her body (and he's very good at that too). His tongue is never still, and it's wicked. On second thoughts, his oral fixation is wonderful. She arches up so that he can take some more of her breast into his mouth. Perfect. Just perfect in every particular. He can indulge his oral fixation as often as he likes. As long as it's with her, of course.

Castle's oral fixation appears to have reached her jeans waistband, and since he hasn't – yet? – developed the agility of his teeth, lips and tongue sufficiently to undo buttons, his fingers have come into play too. He lifts her hips, she rises a little to help him out, and in a trice her jeans are decorating a nearby chair. He growls. (She likes that noise. She'd like it even better if he were a panther, big, black and powerful, but the time isn't right for that just yet. The correct – er – conditions need to be met, and they have not been. Yet.) Anyway, the growl is just the reaction she'd been hoping for when she put that particular pair of panties on. Lace trimmed sin, and Castle never, ever, resists temptation. Not when she's doing the tempting.

She smiles like Lilith and curves like a naughty angel; and Castle growls again and bends down to feast: starting slowly and teasingly, tugging gently at the lace trim and flicking his wicked tongue over the translucent front panel; moving away to tantalise at her smooth cream skin of her thighs, a little nip, a little mark, producing – quite without her volition or permission – a little whimper and then a purr. He looks up, hot satisfaction on his face and sheer lust in his eyes, and licks his lips.

"I love it when you make those noises," he says lazily. "I love it that you're so into it."

She'll forgive him the arrogance. He's so good at making her feel good. She purrs more loudly and flexes like the cat she sometimes is, demanding more. More is exactly what she gets: long, lapping licks and short hard nips; the commanding thrust and swirl of tongue, punctuated by a slide of broad fingers over and round and in; the drugging, decadent sexuality that leaves her taut and clenched and every nerve ending aware of how close she is until that evil, talented tongue and fingers give her soaring release.

And then, of course, it's her turn to play with her prey.

She starts by pulling him up to her, kissing him thoroughly, taking his mouth for her playground and flirting her tongue to his: small indications that it's her turn to make him call for her and give in to her own sensuality. Her fingers run through his hair, petting; but she turns to the nip behind his ear that makes him wholly hers; the languorous lick that leaves him lax in all ways but one. He sometimes tries to take back control of the game, but if she doesn't let him then he's pleased to be petted. He'll make a very fine feline, soon.

She wriggles slowly and sexily over his broad body, ensuring that each inch of torso is touched, dropping tiny peck-kisses down his neck, his throat – she nips there, small predation from an alpha feline, marking her territory and her lover (when she changes him that might change too: she's not at all sure who'll be alpha then, but nor does she care) – a lick around the clavicles, and then attention to his nipples until there are noises roiling low in his throat and he's iron-hard against her.

Further down, and the noises are more of a groan, a little touch of lips against the drop of moisture, a slow stroke up and down, a circle; and more; wicked, delicate fingers and nails reminding him that cats have claws. He's jerking out her name, as desperate as she had been, and she holds him there for another few instants before she lets him fly.

She slithers happily back up his body and drapes herself bonelessly over him, head over his heart, where she's soft and relaxed and herself pettable, until they want to play some more. Which they do, soon enough, slow and gentle and wholly arousing, until she's tight around and he's solid within and everything is simply them.

"I got you a present," Castle says, a day or two later. He smirks. "Well, a couple of presents." He hands over a wrapped box with a ribbon and bow – very fancy – and another prettily wrapped package. Beckett loves presents, though she's also embarrassed by being given things, of which Castle is perfectly well aware. He loves giving her things – Castle loves giving everyone things, which is why even Captain Montgomery is currently sporting a truly tasteless pink stress-relieving squishy pig on his desk (every time he leaves his office someone retrieves it from the drawer in which he hides it and puts it back in the middle of his desk) – but even in the few short weeks they've been properly together, Beckett has had to make it very clear that he is not allowed to shower her with gifts which she is totally unable to reciprocate. She is a long, long way from being a multimillionaire, and she simply can't cope with incessant generosity. Which, she is sure, is at least partly why Castle does it. He only does it to annoy because he knows it teases, the quote runs, and Castle is very keen on both annoying her and teasing her. In all sorts of senses.

However, the package is pretty, and she can always use the ribbon and bow to amuse her Onyx-form. Castle will happily play with Onyx for hours, including as much snuggling and petting as any elegant Siamese cat could ever desire. Cats, Beckett knows, should be worshipped. The Ancient Egyptians had it right. Then again, Castle's pretty good on the worshipping front himself. Mmmmmm.

She unwraps the box carefully, rolling up the ribbon and putting it tidily out the way. The unwrapped box has the name of a very exclusive lingerie shop not far from Castle's loft. She manages to conceal her desire to dive in – her third biggest weakness, after coffee and chocolate, is beautiful, elegant, sexy lingerie: shoes come after that – though she thinks Castle noticed anyway, from his sudden lazy, leonine smile, and carefully extracts the contents.

They are, collectively and individually, gorgeous. Strong, jewel colours; plenty of lace, concealing just enough to tantalise and revealing just enough to tease. Mmmmm. Bra and panty sets, times three; matching garter belts – oooohhhh, those have such an excellent effect – and right at the bottom an astonishingly sexy piece of nightwear. She is so delighted she actually bounces, a little, and purrs happily. Castle looks amused, and happily cuddles her in.

The second box is less gorgeous, but just as much fun, in a different way. Castle had discovered Beckett's well hidden geek-hood rather too early for her taste, but after he'd let on to his own, she'd more-or-less forgiven him. This box contains a set of t-shirts featuring all her favourite sci-fi characters – and where did he find a Lieutenant Chloe outfit? Wow. She throws herself on to him, flattening him (that was entirely deliberate), and kisses him with considerable intent, which doesn't take long to heat up into some exceedingly heavy petting.

The inevitably explosive results of heavy petting having left them both mindless and limp, it takes a little while for Beckett to remember about her lovely presents. She wanders back through to find them, wrapped in a short, skimpy robe which only covers the still-bare essentials. She hasn't forgotten how to model.

So she does model, to considerable, if lingerie-removing, effect. They're even better on than they looked, though they don't stay on for long. By the time she's finished, even Castle simply admires the nightwear, and falls asleep with her with his hand very possessively over her hip, stroking the soft chiffon.

Beckett wakes first in the morning, and after some much needed ablutions, puts the pretty piece of temptation back on and slips into bed, turning into Onyx to tuck into the warm notch between Castle's neck and shoulder where she fits. The nightwear shifts with her. Very secretly, she is a little pleased. She had wondered – though that shop is very high end – if he'd been intending to do some sneaky theory testing. He'd been remarkably silent on the subject, where normally once he has a thought in mind she hears about it non-stop until she either caves or draws her gun. She snuggles in and closes her eyes again, purring softly until sleep overtakes her.

She is woken, rather later, by Castle petting her fur. "Hey," he says gently, big blue eyes looking straight into hers, "it's morning, sleepyhead."

Beckett emits a displeased feline noise, and not accidentally twitches the end of her tail to hit his nose.

"Not nice," he chides. "I was petting you."

Beckett changes back. Castle's eyes light up at her covering. "I was asleep," she points out. "My weekend off. I sleep."

"You can sleep with me," Castle rasps, and proceeds to prove the point, extensively, after which Beckett does indeed fall asleep again. So, indeed, does Castle.

This time Beckett wakes first, and takes some justifiable revenge by washing, brushing hair and teeth, and generally getting ready for the day, rather than staying snuggled up in any form. Castle, when he wakes (probably because the aroma of coffee is swirling through the apartment), is not wholly impressed. He's a little less disappointed when he realises that Beckett hasn't actually bothered to dress yet, preferring a soft cotton robe.

"Are you going to show me the t-shirts?" he asks hopefully.

"Might do," Beckett teases, "if you're good."

"I'm very good," Castle preens. "You love how good I am."

"Then you might get the t-shirts shown."

"Mmm," he hums. "C'mere." He reaches out apparently lazily and traps her in. "Mine." Quite without asking, he inserts her into a t-shirt. It has a picture of Elektra across the front, in manga style. Beckett tugs hopefully so she can go and inspect it. Castle doesn't let go. She takes the obvious option and changes form, slips away and then changes back.

"I like this one," she says. "But you're being childish not letting me go look."

"I like it when you're trying to escape and can't. All that wriggling gives me all sorts of ideas," Castle leers cheerfully, attempting to look piratical. Beckett rolls her eyes. "C'mon. You love my ideas." She slinks back and reposes upon the couch again, which swiftly becomes on his lap. "Next one," he decides, and the process is repeated a couple of times.

The next t-shirt has a picture of Dangermouse. Beckett looks at it, and looks at Castle, who shrugs.

"Thought it was appropriate," he says. "Private joke." He puts it on her. Beckett essays an escape attempt, fails, and changes. The t-shirt does not change. The t-shirt, in fact, has draped around her. She growls viciously, and would swat Castle, except that the t-shirt is in the way. She is not at all happy, and bundled up in the – clearly non-natural – t-shirt, she can't even scratch. Her growl acquires a more dangerous edge.

Castle does not appear to care. He is grinning widely. "I thought so," he says smugly. Beckett can feel a drape of fabric dangling from her ears. It doesn't improve her mood. "Less than 50% natural fibre and it won't shift with you." Beckett declines to change back and participate in this discussion. She is not at all happy that Castle has experimented when she didn't want to. She lays her ears flat and hisses, but declines to run her claws out (and preferably into Castle) on the grounds that they'll get caught in the t-shirt.

"C'mon," Castle says. "It's important to know the limits."

Onyx-Beckett ignores him.

"Now I know what not to get you."

More cold-shouldering. She presents him with her rear and a very offended tail, and remains under the horrible man-made fibre t-shirt. She detests looking foolish, and Castle has made her look foolish. Cats do not appreciate being made fools of. She would wash her paw, icily, but she is not doing that when swathed in a cheap, nasty t-shirt.

Which is when Castle cheats. Again. His hands sneak under the t-shirt and shortly prove to be holding a brush. Onyx-Beckett ignores his smooth, sweeping strokes, which only causes him to brush more. She is not impressed – with her own inability to resist. She's also not impressed by Castle's subverting of decent behaviour – that would be not cheating – to achieve his own ends – which would be coaxing her out of her extremely bad mood in the single way that's absolutely guaranteed to work – sexuality. Humph.

Her conviction that Castle deserves to be thoroughly brought to a sense of terror becomes entrenched. Before the brushing can melt her into mush, she contemplates. Hallowe'en is only a week away, and she's already checked both the likely weather – clear skies, unusually – and the phase of the moon – full. She'll only get one chance at this, so she has to take it, but that doesn't mean that she can't enjoy terrorising him first. She is definitely not going to explain, or prepare him. Serves him right.

Her purr is at least as much satisfaction at the thought of how he's going to react as it is sensual pleasure at how he's brushing her. At first. Her revenge amply planned, she relaxes into the strokes of the brush and extends bonelessly over his lap. Not long after that, she allows Castle to discard the t-shirt and then shifts back to become a naked Beckett on his knee, which is pretty quickly translated into a naked Beckett and Castle in her bed, where she makes it very clear that Castle has to earn his forgiveness. She does like a man who wholly enjoys his work, and his tongue works very hard indeed. So, too, do other areas; until they are momentarily sated and content.

Castle mumbles darkly as she slithers off to clean up a little, and mumbles more as she takes some time about it. Mumbles are replaced by jaw-dropped drooling when she walks back in – in the cosplay outfit. Half a millisecond later there's a predatory growl and grab and the brave Lieutenant Chloe has been captured by the big bad wolf. Strangely, escape is not the first thing on her mind, and being eaten by the big bad wolf does not involve pain.

Afterwards, when she cuddles up and turns into Onyx again, there aren't any clothes about which to worry, and right now she couldn't tell the difference between artificial and natural anyway. She yawns widely, and contemplates her revenge.

Castle might be keen on unnatural fibres, but she wonders just how he'll take to a whole unnatural life.

Thank you to all readers and reviewers.

This, as noted above, fits in the time jump in Chapter 6 of Felis Felix. You will note it is a one-shot. See, I can write short!

Next up in this universe, a two-shot starring O'Leary, with a little Caskett.