You take the costume, and look for the changing rooms. Once said changing rooms are found and you are inside, you look at the costume in greater detail. You marvel at details you hadn't even noticed before.
The outside of the costume is soft with realistic fur, and the inside is inside is smooth, like latex. "Kinky," you think.
You can't wait to try it on, so you quickly remove your clothing down to your boxers. As it's all one piece, with just a slit in the back, you lay it out with its back facing upward, and sit near the feet. Scrunching up the legs of the costume so that the feet are practically connected to the lower torso, you slip one foot then the other through the slit in the back and into the corresponding paws.
"Ooohh, that feels good." You stop for a minute. You don't remember saying that, but maybe you're mouth got ahead of your brain momentarily. You press on, pulling the legs up over your own. It's a tight fit, but it seems as though the costume was custom made for you, it fits so well. Nearing your buttocks, you see a problem. The costume has just barely been fitting over your body until now, and the boxers would be fairly obvious through the costume. You pull the costume back off your legs, or rather, you WOULD have pulled if off your legs if it wasn't so tight. You can't get it to budge an inch. "Let meeeee help you with that," you hear, looking up expecting to see some weird person looking over the changing room walls. "Down here." You look down, but instead of someone peeking under the walls, you see the costume mask, seemingly hovering in the air, turned around looking right at you. It's left paw raises up, and you see claws extend. Panicked, you try harder to get the costume off. The clawed paw sweeps by your abdomen. Instead of searing pain, you feel a slight breeze in your nether regions. Looking down you find that it only cut off your boxers. Semi-releived, you now go back to the impossible task of getting this costume OFF. It looks up at you. "Now, wheres the fun in that?" It's arm stretches and grabs your arms, wrapping around them like furry, latex handcuffs. With your arms bound in front of you, the costume then begins to pull itself up your body, covering it as it goes. Reaching your upper-chest, it releases your arms and quickly subjugates them into it's own repituar of covered flesh. Up to your head now, you find that you have no control over your lower body. The costume reaches up with your arms, and puts it's head over yours. The snug rubbery mask goes over your head tightly. It reaches back and runs it's paw along the slit in the back of the costume. You can feel (and just barely see in the wall mirror) the flaps melt, then grow together, reforming. Suddenly you feel control over your body return to you, and you claw/paw at your back, trying to rip open the seam again, but to no avail. You begin to think on how to get the costume off, but you're interrupted by the feeling that the costume is getting tighter. You feel momentary pain in your feet, and look down to see your feet lengthening along with the costume into longer, more feline paws. Now you can feel the carpet with your feet (paws?). You run your hands (paws?) over your body, finding you can feel everything. Your paws (hands?) find a lump just above your rear. Looking back, you see the fur and skin (costume?) liquidize, and grow out into a long tail before solidifying again. You wave your new tail experimentally. Remembering something else, you look between your legs and are disappointed to see only furry skin over a lump. As you watch, that too becomes liquid, showing in much more detail a sheath with balls underneath and becomes once more fur and flesh. You turn to-wards the mirror, and open your mouth (muzzle?) experimentally. You see large fangs, and a much larger mouth on the inside than you would have expected. You close your feline muzzle (mouth?) again, to find it has lengthened slightly into a short extension on your face. After a few more seconds of no apparent changes, you decide to experiment. You try to turn a stubby finger on your left paw into liquid. It works, and you quickly turn it back. "Hmmmmmmm," you purr. That gives you an idea.