The transporter must have malfunctioned, for Spock is distinctly aware of the sensation of shrinking as he materializes in the holding bay. (As per protocol, Mr. Scott has materialized him in a quarantined holding bay as opposed to the transporter room, which is typically more difficult to secure.) There is a 0.92 percent chance that Spock has been exposed to an alien virus while among the Caitians - anything above 0.01 percent requires quarantine by default - and that, combined with the possibility of a transporter malfunction, is the most likely explanation for Spock's current condition. There is no need for concern; this is a problem that should be easily rectified once its cause is logically deduced.
He steps out of his uniform; it has collapsed around him in a heap of untidy fabric, and he shakes it off with his… paws.
Spock freezes, caught mid-step as he stares at his newly acquired appendages - a pair of paws where there had once been hands. Spock lifts one, inspects it, and then lifts the other to inspect it, as well. He is possessed by a peculiar urge to lick it - it smells of graphite and the vaguely organic dust of the holding bay - but he overcomes that urge with the ease of long practice. It is no more difficult to overcome than the occasional violent urge (recurring approximately once every 1.38 cycles) to throttle the captain in his sleep.
It appears that Spock has been transmuted into a small mammalian creature. His fur is a sheer near-black, burnished to an aesthetically pleasing slate grey by the overhead lighting, and Spock is - startled - at this flattering self-assessment of his own form. He cannot recall being so invested in his appearance when he was a humanoid; he barely paid attention to anything other than the neatness of his uniform and the maintenance of proper hygiene.
His hind legs are short with pronounced haunches, and are also furnished with paws. He leaps onto the nearest crate to test his range of motion, and is satisfied by both the distance and the alacrity of his leap. He also seems to be in possession of a lengthy but nonetheless non-prehensile tail; it flicks when he pays attention to it, and, once again, he has to fight the urge to pounce on it. Its movements are… distracting.
The logical approach would be to ignore it, so that is precisely what he does. He focuses, instead, on the far more productive line of mental reasoning that calls up and discards innumerable entries from his internalized database of genera both Vulcan and alien.
Via a process of simple elimination, Spock concludes that he is a cat. A Terran cat, moreover, although he cannot be certain of the specific species until he has more data.
The Caitians almost certainly have a hand in this.
Well, a paw.
Having reached an adequate (if temporarily incomplete) conclusion, Spock hops back onto the floor and pads over to his crumpled uniform.
"Spock?" crackles his combadge; it is Captain Kirk's voice, somehow louder and more piercing than it has ever been before. Spock can detect in it a note of (illogical) concern. "You okay? You usually check in the moment you're back, but - "
Spock presses a paw to the combadge and… mews.
There is an echoing silence from the other end of the combadge.
As Spock had not expected to be capable of speech in this body, he is significantly less surprised.
"…the fuck?" says the captain, as inappropriate as ever. "Bones, run a goddamn diagnostic, would you? Get us a visual on - ohfuck. Fuck. That's a - is that a cat? What even - "
Spock mews again, attempting to imbue the sound with authority, useless as the effort may be.
"All right, all right, we're getting you out of there. Are we in the clear, Bones?"
"Ain't no virus or bacteria or freakin' nanobots on 'im, more's the pity," growls Dr. McCoy. His faux-aggression towards Spock is puzzling, as always; the doctor has saved his life often enough to render any threats he makes utterly invalid. The Human tendency to adopt apparent emotional states that are at odds with actual emotional states is both inefficient and unfathomable, no doubt a throwback to more primitive evolutionary means of camouflage. "And that's Spock, all right; it's obviously his DNA that's been munted, and Scotty here keeps saying he's sure he'd fixed on Spock's signature, not some random animal's."
"So he's a cat." Kirk's voice is quivering strangely. "A cat."
Spock opens his mouth to inform his captain that he is unnecessarily restating an obvious fact, but all that emerges is a another mew.
Kirk snorts. And breaks into a series of odd, syncopated noises.
Is he - is he laughing? What exactly is amusing about this situation? Not that Spock has ever admitted to understanding Human humor, but -
"C'mon, Jim. That's cruel. You've got the poor little kitty hissing - "
Spock hadn't realized that he had begun hissing; he attempts to stop it, with little success. McCoy's sly, patronizing tone certainly isn't helping; it is reminiscent of the very tone Spock's fellow Vulcan students had once used on him.
"Fine, fine, just - calm down, Commander. Heh."
Spock… does not growl, for all that his hackles are up. He takes a deep breath, regulates his heartbeat, and calms himself. There's no need to make even more of a spectacle of himself than is unavoidable. It is already untenable that he should be unable to perform his duties; while in this feline body, he'll have no meaningful contribution to make. Unless…
"Looks like he understands us. Before I go down there, Bones, what kind of cat is he? Hopefully not some psycho, man-eating variety?"
"Nah. He's a tame ol' Russian Blue, according to the computer, but there're differences. His fur's a couple shades darker than it should be. And his eyes aren't a normal cat's eyes; they're trichromatic, closer to a human's. And he's got weird psi-waves. He's probably still telepathic."
Interesting. As he had hoped, he has retained his telepathic abilities; if he's capable of touch-telepathy, Spock might be able to meet at least some of his dual responsibilities as First Officer and Science Officer. He will likely be the first feline officer in Federation history. (No, he will definitely be the first feline officer in history; the Caitians are part-humanoid, and do not count.)
Also, the fact that his transmutation has been selective indicates a degree of deliberate genetic engineering, rather than an accidental 'SNAFU', as the captain might call it.
The Caitians have just committed a biomedical crime as well as a grievous diplomatic error. Their motives are as yet unclear, but once returned to his true form, Spock will demand all the pertinent information, as well as legal redress. If he can obtain a significant sum of (entirely justifiable) monetary compensation, then he will be able to help the Vulcan colony fund its new science station.
"Hang on, Spock," says Kirk. "I'm coming down to getcha."
Spock purrs in response, folds his paws over each other, and waits.