Looking around in the cupboard, you find a bag with some unusual items in it. Emptying it out for a look, you see that inside are the elements of a catgirl costume. Hey, why not? you figure; as long as you're experimenting, you might as well give it a try. Carefully parting your hair from front to back, you put on the headband with the cat-ears on it, then rearrange your hair to cover the band. Next comes the belt, which you fasten around your shapely waist; once it's in place, the tail hanging off of it is positioned so that it actually looks like an extension of your spine. Reaching into the bag, you dig out a set of novelty fangs, which you place in your mouth, grinning. Then comes a pair of gloves - the fingers are a little stubbier than normal, (though still perfectly usable,) giving the whole thing the appearance of a cat's paw, especially since the fingers are tipped with claws, and the palms and the undersides of the fingers are padded. Finally, there are a pair of socks much like the gloves, which are shaped like real cat's paws; amazingly, they fit perfectly on your feet.
The whole ensemble looks reasonably good, though you can still tell it's a costume. Although it's quite pleasant looking at a beautiful girl in her underwear in the mirror (though, strangely, not a turn-on as you'd have expected,) you decide to put some clothes on. Rummaging around in the closet, you find a pleated blue denim skirt and a white blouse. You put on the blouse first, then, reminding yourself that you're now a girl so it's okay to wear one, you step into the skirt and pull it up to your waist; it comes down to a couple inches above your knees.
The entire outfit looks remarkably good on you, and you stare at your reflection, pleased with yourself, until there is another tingling sensation. You brace yourself for the worst, but there is only a pain in the sides of your head. You clap your hands to your head, only to find that...your ears are gone!?
You remove your hands and take a look. Yep, they're gone; there's only smooth, unbroken skin on the sides of your head now. Realizing what must have happened, you tentatively reach a hand up to your ears. Yep, they're real. Quick checks confirm that the rest of the costume is real as well; your canine teeth are now full-fledged fangs, your tail is real, your hands are now somewhat pawlike, your feet are full-fledged paws, and both your fingers and toes are tipped with retractable claws. All your cat parts are now covered in orange tabby fur, which complements your long red hair quite nicely; the fur on your hands and feet has extended midway up your forearms and shins.
You stare at the girl in the mirror. You're not just a girl anymore - you're a catgirl.