...Maya Fey costume you had bought for your now ex-girlfriend a few years ago. The costume barely covers your naked butt, but you slip it on regardless. You stand there, a grown man in a very poorly fitting woman's costume for a few moments, before you notice something odd. Is the room...growing larger? You look down and realize that the opposite is true: you are shrinking. That's not the only thing you notice. You brush your growing hair out of your eyes as you stare at your skin. It's becoming softer, the hue changing subltly. You can't seem to feel any of the changes as they happen, only by looking down are you noticing your thinner waist, your growing hips, your expanding breasts. Before you can really register what's happened, it's over. Maybe you should look yourself over, see what's changed.
You take a deep breath, ready to see what exactly just happened to you. You try and steady yourself, still not acclimated to your new body, and look down. Before you can really take note of anything, a lock of long black hair falls into view. What? You've always had brown hair, not black! You touch it, and it feels soft and silky in your hands. Speaking of which, your hands look a lot smaller than before, and your nails are longer. You hesitate at calling them feminine, if only because you're not ready to accept that yet. You touch your face, and it's clear your nose is much smaller, like a button nose. Your lips are the only thing on your face that haven't shrunk, they're slightly larger than you're used to.
You reach down, to your neck, and freeze when you realize you don't feel an Adam's apple at your throat. There's...no way. You can't just...change like that, can you? Realizing you might as well get on with the inevitable, you lower your hands to your chest. You find what you feared: breasts. They're not exceptionally large, in fact they're a little small. I guess it fits your small stature, you think. Though you do wish they were a little...larger. Why did you get stuck with mosquito bites?
Wait, what? Since when would you want larger breasts? Until today, you didn't have breasts, period! Maybe this transformation is affecting your brain too...Well, you still need to figure out the rest of this change, right? You haven't looked over your whole body yet. Yeah, that's right. You move your hands down, further, to your waist. It's definitely smaller around than it used to be, but your stomach is mostly unchanged. The only difference there is it went from an average guy stomach to a cute, girly stomach. Your hands keep wandering south, feeling around your hips. They didn't shrink nearly as much as the rest of you, which means that with your smaller frame they come off as a little bit wide, giving you a slight hourglass figure.
The more masculine part of you can't wait anymore, and you reach around to grope your butt. It's definitely a little bit bigger than it used to be, but for a girl it's not really that big. Maybe if you eat more, you can work up to a sexy bubble-butt! Of course, that would mean being in this body for quite a while. But...that doesn't sound too bad. You shake your head, trying to clear your thoughts. There's really only one thing left to check. You pat your crotch a few times, and it's clear that your old equipment is gone. You are 100% female now.
You turn around in a circle, trying to get a full look at the new you. As far as you can tell, you're the spitting image of Maya Fey. How did this happen to you? Maybe it was some kind of crazy new super-drug! Or magic! Or aliens! Or...maybe it was whoever took all your stuff. Maybe you should check that note again.
"Dear Mr. Cobb,
So, you've noticed your new power, eh? Now, whenever you try to wear clothing that doesn't fit you, you'll change to fit the clothing! Right now you're stuck as little Maya Fey; not even wearing other clothing will change you back. If you want your stuff and your original body, you need to get yourself to the convention near your apartment complex, and win first place in the Cosplay Contest. The prize is a key that unlocks this box. Good luck.
Sincerely,
Contract Providers"
"P.S. Be careful with your choices from now on. Stray thoughts about how you'd like to change can easily become a reality. All further changes will make your mind slip further and further into the Maya persona. If you fall too far before getting the key, there will be dire consequences."
How did the note change? You've been in the room the entire time, it hasn't moved, and no one has come in. You guess magic must be the answer, right? Regardless, you need to get to the convention. If the times for the events are anything like what they were last time you were there, the competition should be starting soon!