You, of course, should not have played around with a new, world-changing invention without at least finding out how it works, but in this case the subtle nuances of Dr. Marks' last invention saves your life. Just as you're passing out from a lack of oxygen while shoved deep up the giant Mary Lynn's pussy, a failsafe procedure on the gun begins to take effect. Although all you thought you had done was shrink yourself, the gun also gave you the property that, should you approach death, certain protocols would change your form until it found a way for you to survive. Here, there wasn't much it could turn you into where you'd be able to breathe, but there was a protocol for dying inside another living being.
You, of course, don't find out any of this. All that happens to you is that you pass out. Then all of a sudden you're fine. You're sitting in a restaurant, though you don't know the people you're with, and, wait a second, you're back to normal size. How did that--wait a second, this isn't your body at all. This is a woman. Holy shit, you've turned into Mary Lynn!
"I'm--uh, I'm feeling, feeling sick," you stammer out, shocked at the sound of your own voice. "I'm, I'm, I'm going to step out for a second." You get up and start walking, which itself feels weird inside the body of a differently-shaped person, but you can manage it. You find the restrooms and have to stop yourself before going in the men's room. You go one more door down and enter the women's room.
Thankfully, you're alone in there. You look in the mirror, and sure enough, you're Mary Lynn, a middle-aged librarian. You look at your own chest in amazement. Although Mary Lynn is obviously clothed, you can make out the faint outline of the black bra you were trapped in for half the day. You then look down at your crotch, the hot prison where Mary Lynn had kept you after lunch. Everything looks so strange from this perspective. It's like everything is normal sized now, but you're used to being tiny now, so it's weird to look at. You grab your boobs. This is so strange.
You stare at the mirror for a while, and then your expression changes to pure confusion, without you doing anything. You find yourself starting to turn away from the mirror and back towards the door, which freaks you out a little, but you're able to stop yourself. Mary Lynn must still be inside this body too. But you're able to move her body freely when you put your mind to it, so your decisions must be superseding hers. That should be fine, as long as you keep paying attention.
You look yourself in the eyes in the mirror. You know Mary Lynn can hear you. "I came to you for help, and you treated me like a toy. First your bra, then your panties, then crawling up your pussy. Now I run the show, and I'm going to for as long as I want."
You maintain your hard stare, so the real Mary Lynn can't do anything in response. You smile. The bitch deserved it.