"So let me get this straight, if you had the chance to be a shapeshifter you'd take it?"

"Yes! Sure I would, Beckett. It would be so amazingly cool. I mean, I'd be this huge, powerful animal" –

"What if you were a squirrel?"

"No, no. That's not in the rules. You've got to be some alpha predator type. Wolf, or lion, or tiger. No-one's ever a squirrel." He makes a plaintive face. "I wouldn't want to be a squirrel. Some oversized dog would eat me." Beckett snickers. "Well, unless you protected me."

"I don't think squirrels make good pets, Castle. They might be small and fluffy but they're a pest, not a pet. I wouldn't be protecting any squirrels." Definitely not. There is only one use for squirrels and it certainly isn't as a pet. A snack, now…

Castle pouts. "I wouldn't want to be a squirrel anyway. Not macho at all. Not even metrosexual."

Beckett snickers some more. "Not suitable for your ego, then?"

"Not suitable for my ruggedly handsome physique." He smiles, slowly. "You wouldn't like it if I were a squirrel." The smile slips into lazy predation. "You'd like it if I was the big bad wolf. I'd eat you all up."

"In your dreams, Castle. In your dreams."

Beckett returns to her case files, pondering. Apparently she is pondering the case, to all external observations. In fact she is pondering next steps. She has a secret: one that she's never revealed to anyone at all, and it's getting pretty lonely out there.

But. But she is not sure of Castle's feelings, at all. They've been quite cautious about each other, even though they've managed to patch matters up since the disaster of the summer, and it's almost back to good.

She ponders some more, after he's left, as she makes small notes about the case and possible leads. And suddenly it occurs to her that she could very easily find out how Castle felt, at least about one matter. He told her that has a reading tomorrow night, at the Rizzoli Bookstore, and she can sneak into that. Or, more pertinently, sneak around him. There's always a break in the middle, and she has detected that Castle usually needs a few moments out of the limelight (astonishing, but there you go). In that break, she might make some overtures.

And most importantly, she can find out if he likes cats. Given that she is a cat, that's pretty crucial.

"But it's not fair, Beckett," Castle whines. "I wanna know how you do it."

"Not telling," she says childishly. Castle pouts, equally childishly.

"You don't tell me anything. You're keeping secrets from me. I wanna do it too, and you won't show me how."

"You experimented with t-shirts and made me look silly. Anyway, I've answered all your other questions. There's only me. I'm unique."

"Must be lonely," Castle notes, a little more seriously. "If I was a shapeshifter too, you'd have company."

"Yeah, right. I've got your company." She smiles extremely seductively, and pops a button on her shirt. Castle stares at her, suddenly completely distracted. It's downright wonderful how he looks at her when she does that. Now… ah yes. There we are. Castle has scooted across the couch (he shouldn't have been sitting at the other end anyway) and without her even getting to a second button is proving how much he appreciates the invitation.

He tickles along the seam of her lips, encouraging her to open to him, and when she does surges inside to raid and possess and conquer. Well, he tries. Love is a battlefield, after all, and she's not inclined to let him win the war without a fight. She raids and possesses on her own account: heat rises as she pulls away and resettles her mouth on the nerve behind his ear where one small nip (and she intends to test that feline technique upon him much more often) has the most interesting results. Just as he has before, he growls low in his throat, which vibrates all the way down Beckett's nerves and leaves her damp and quivering.

She stretches out and, cat-like, offers up her throat to his most excellently mobile mouth. Not that this means she's stopped playing, of course. His buttons require to be undone, revealing that lovely broad chest, so particularly perfect for snuggling on either as Beckett or Onyx. Of course, it won't fit her panther form, but that's okay. She's pretty sure (despite never having done this before) that he's going to be a very sizeable cat (or panther) indeed. Her cats, after all, reflect her slim elegance. It makes perfect sense that his will reflect his impressive size.

Currently, however, said impressive size is right there beside her and gently easing his way down into the vee of her shirt. She doesn't have to pop any more buttons. That's all being done for her. How very convenient, since it frees up her hands for some mischievous marauding of their own.

Much to her irritation, Castle is getting in the way of her manipulations. He's managed to nibble down just far enough that she can't reach his waist any more without trapping her hands between them. She doesn't want her hands trapped, she wants them to be free to do delightfully provocative and wicked things which will leave Castle a melted mess. But she can't reach that far down any more, and Castle is pillowed on her in such a way that he is preventing her turning him into a puddle while being perfectly positioned to turn her into a puddle. Humph. Not that she objects to puddling, but she likes to have the first go. Unfortunately Castle is equally keen on having the first go, which frequently leads to competitive arousal.

"Now," he rasps, "isn't that pretty?" He smiles lazily. "Let's make it a little prettier still," and her opened shirt becomes an absent shirt. Despite the lazy smile and tone, his eyes are intent and hot. "This is pretty," he muses, and traces a thick finger very delicately along the edge of a rather revealing lacy cup: a whisper of sensation through her breast that shivers through her nerves. The barely-there touch ghosts along the line of fabric, rests for an instant in the valley of her cleavage, and ghosts again across the other side. Her nipples peak, though he hasn't touched them. Her core is moist, though he hasn't touched that either.

His wicked fingers continue to stroke, round and about, closer and closer to the hard pink points, gradually firmer and firmer, becoming soft palming and then moulding and then hot breath over the thin lace. She arches up in sensual suggestiveness and uses those thankfully free hands to try and bring his mouth down to where she wants it. Annoyingly, it doesn't work, which is very unfair.

"Tut-tut," Castle chides, though it still leaves his breath ghosting over the fabric. "Anticipation is the spice of life."

Beckett growls. At least she means to growl. It's at least half a growl. If Castle wasn't seducing her it would definitely manage to be a growl. How did it start off her seducing him and now he's turned the tables? And she can't even raise a growl because it's now at least three-quarters a purr.

It's just not fair that he's this good at seduction, no matter what form she's in. He'd seduced her out of her annoyance about the t-shirts, and now he's no doubt planning to seduce her into compliance with some ridiculous plan.

Oh. Humph. He's stopped. That's not allowed. He's propped himself up over her stomach and is simply admiring. Which is very nice, but his eyes are no substitute for what the man can do – and should be doing, for that matter – with his mouth. Or his fingers. Humph. Well, if he isn't going to do anything, she is. Especially now that she has some room to work.

Her hands sneak into the space that Castle's left between them, and manage to complete untucking his shirt, preparatory to pushing it off. Since pushing it off is not practical at present, and tearing it would be unkind and more pertinently require a swift change to Onyx to take advantage of sharp claws, which would be a considerable disadvantage when it comes to the next stage of affairs, she wriggles just a little in a thoroughly effective fashion – oohhh yes, just there – and then manages to undo belt buckle, button and zip in a rapid raid. Then, of course, she's free to whisk her hands out from between them (not without a very naughty stroke and swirl) and use trained strength to loosen his pants. Perfect. Her hands slide over his well-formed ass, and squeeze. Purely for encouragement.

Castle growls. Beckett wasn't encouraging him to growl, she was encouraging him to carry on with his deliciously erotic attentions. On the other hand, if he's going to growl, she'll undertake some erotic attention of her own. His pants hit the floor, and his back hits the mattress. Beckett slinks over him, and produces a predatory, sexy smile of her own.

"My turn," she husks. "Now, where was I?" She scrapes the edges of her nails down his pecs, and follows with the very tip of her tongue. Castle tenses, so she playfully pins his hands down (that won't work when he stops letting her do it) and carries on. His commentary is extensive, though punctuated by groans, and she's barely even started on the – er – hard stuff.

Castle might have a serious oral fixation, but her tongue is pretty talented too, as are her fingers. Her tongue stays playing around his flat nipples, a little twist, a tiny nip, a small lave; but her fingers stroke downward across his skin. Of course, letting go of his hands does free them, of which he takes considerable and immediate advantage: her pants come to rest halfway to the door, the hems kissing his own. She slithers to one side, for better access, and flitters her fingers down, following the dusting of hair to his navel, below, avoiding his hard arousal, traces around the base and behind, soft skin tightening under delicate touching: his hands come up but she's already trailing down, hair skimming his skin, lips barely against his body, all the way down.

Her tongue slips out, and licks, once, straight up. She doesn't get a second chance. Castle growls extremely dangerously, flips her over and pins her down on her back, hands pinioned by her head, and doesn't pause for an instant before making it perfectly clear with every commanding, dominant thrust of his tongue and grind of his hips that he's going to be in charge of the playground. Just as she would in her big-cat form if he were her alpha male, she succumbs to his force and his size and moves encouragingly beneath him.

Encouragement results in her wisp of panties departing at speed, her bra following, and Castle's hard thick weight settling against her, sliding slickly through the folds: he's still ravaging her mouth with forceful possession, pressing down and covering her and finally, finally, filling her and taking her and completely, totally hers.

Later, still tangled together, Castle's idly stroking her hair just as he'd stroke her as Onyx.

"Why can't I be a shapeshifter too?" he says wistfully. "Then there'd be two of us."

"Thought you wanted to be some sort of huge predator?"

"I didn't want to be a squirrel. If I was a cat like you" – you don't know what you're asking for, Castle. If you only knew – you'd be even keener than you are – "though I'd be bigger," he says smugly – "then you wouldn't be lonely."

"Lonely? I'm not lonely."

"Well, no, you've got me, but wouldn't it be so much nicer if I could shift too? You could pet me, or we could… we could… we could chase mice together," he says happily.

"My apartment does not have mice," Beckett says crossly.

"No, that's because you've chased them all away." She growls, not notably impressed. Her apartment certainly does not have mice. Her panther form had done a sweep, when she moved in, and comes out to stretch every month or so.

"Anyway, wouldn't you like having someone else who could change?"

"Are you really that desperate to be a shapeshifter?"

"Yes!" Castle says emphatically. "I'd love to be able to shift form."

"You wouldn't ever be able to tell anyone."

"I can keep a secret," he says, offended.

Beckett raises an exceedingly cynical eyebrow. "Really?"

"I haven't told a soul that you're Onyx."

There is a pause, while Beckett considers that. It's absolutely true.

"I have no idea how to teach you to change," she says. That is also absolutely true. She has no idea how to teach him. On the other hand, she has an extremely good idea of how to change him.

Castle droops. "But I really want to," he says plaintively. "Even if it's a cat, not a lion or tiger or" –

"Bear?"

"Oh my," Castle sniggers. Beckett smirks. "But I do wanna, Beckett. You should want it too."

"Why?"

"Well," he oozes, then smirks, "you wouldn't always have to change back after I've brushed you…"

Beckett thumps him, without much force. "Do you ever think about anything else but sex?"

"Yes, but not when you're naked in bed with me."

He rolls over and kisses her, stifling any further comment for quite some time.

"If I work it out," Beckett says sleepily, much later, "do you really want me to show you how?"

"How many times do I need to agree? Yes!"

Three times will do just perfectly, Castle. Three times is the charm, and that's the third consent. You really do want this.

And you're going to be so surprised when you find out about the third shape…

Hallowe'en night

Beckett slithers off the bed, ostensibly to go and clean up. She will clean up, too, so it's not untruthful. Castle is sprawled out across the bed, completely sexed-out and delightfully exhausted.

Step One: complete. He's totally ready.

Conditions: perfect. Hallowe'en night; a full moon; her partner thoroughly satisfied.

Beckett pulls the bathroom door to – she can't close it, because paws don't work so well with doorknobs and she's never had a reason to change the bathroom door to a lever handle – there is a soft, inaudible sigh, and suddenly she's her panther form: as ebony as Onyx; pure midnight lethality.

She opens the door and pads out. Castle half opens his eyes – then jerks to sitting and shrieks. She keeps padding: slow, metronomic steps. He's paralysed. She leaps up, pins him down and pierces the sheet with her claws – dammit, she didn't mean to do that.

"Beckett? Beckett? Please tell me you're Beckett? This isn't funny, Beckett! BECKETT!"

She emits a coughing laugh and bends her head to his neck, sharp teeth against his skin, not quite drawing blood.

"Curiosity killed the cat," she says – hang on, she's never been able to speak in shifted form: clearly there's more than one magic spell operating this Hallowe'en – perfectly clearly through the open jaws around his neck, "but satisfaction will bring you back."

She bites down, through his jugular.

"What did you do? You bit me! And why did you never say you were a jaguar? Why'd you bite me? It hurt, and you scared me, Beckett. That's mean!" Castle's incoherency is in full flow. He appears to have missed the point.

"I'm a panther. It's different." Castle doesn't appear to appreciate the difference. "You're supposed to be scared of me, and anyway you made me look silly with those t-shirts when I didn't want to experiment."

"You scared me 'cause I embarrassed you? That's not fair. And you bit me."

"Haven't heard you complaining about me biting you when we're in bed before," Beckett smirks nastily. Castle's ears turn red. "Anyway, you said you wanted to be a shapeshifter."

"You bit me and it hurt! It doesn't hurt when we're in bed. And you were terrifying though I have to say that the panther is much more like you than a cat – what?" His brains have finally caught up with his ears.

"You said you wanted to be a shapeshifter."

Castle stares at her. Then he acquires a most unusual expression of concentration – and suddenly there's a full-grown, massive, pure black panther lying on Beckett's bed. He smiles, and sharp teeth gleam. Beckett smiles back, and stretches out a hand to fondle his ears. Castle-panther emits a deep rumbling purr, and butts his head into her hand for more. The purr becomes continuous.

He changes back, concentrates, and is a large, black domestic cat: much bulkier than Beckett's own slimly elegant Siamese.

"Look at me!" he says, human again. "Look, I can do it!" He sounds like a four year old mastering a climbing frame. He cycles through the three forms another time, and then, human, pounces on Beckett and pins her to the bed. "You're mean," he says. "You knew you were going to scare me."

"Yep," she grins. "But you agreed to become a cat. Three times."

"Why'd you never show me the panther?"

"Keeping her for a special occasion," Beckett says lightly.

"Mm," Castle hums. His tone shifts as he does – and suddenly he's a panther again. That's not going to work. Beckett shifts herself to Onyx and scampers away, stopping by the door and inviting him with a flirt of her tail to play chase. He's after her in an instant, as comfortable on four huge paws as he would be on two feet: size making up for his lack of the Siamese's ability to slide into smaller spaces.

Finally he traps Onyx under one large paw: lies down and very carefully nips gently at her neck. Onyx exposes her throat in feline submission; Castle-panther releases her, and she snuggles into the space between massive forepaws, where, she finds, she fits perfectly. She purrs at him, unsurprised to find that she's no longer able to talk in feline form. It's after midnight, and Hallowe'en is over.

Onyx, delighted with her playmate, nips his neck, changes back to Beckett and finds, even more delighted, that Castle-panther is as silky-softly furred as she could have hoped. She strokes him for a while, stretched out next to him. His eyes haven't changed, still the same bright blue. Still full of love. She plays with his ears for a while, and listens to the deep purr and notes some very obvious arousal. His ears are clearly just as erotically charged as hers.

Finally he shakes himself out, and changes back.

"You did it. You really did it. I'm a shapeshifter too! Oh, Beckett," he says, utterly happy, totally adoring and adorable. "You couldn't have given me a better Hallowe'en present."

"Yep. Two of us."

"It's going to be so cool, as cats."

Fin.

Thank you to all readers and reviewers.

Some of you wanted to see how Castle got to be a panther. During Unnatural Fibres, some of you were worried that Beckett was going to change him without his consent (absolutely not, as you see).

Next up in this crazy universe, Toddler Taming. Back to Caskett and their twins, and the trials of parenthood.