First introduce a few things or people that might get vengeance exacted on it later.. Mom with a frustrating ex-husband who is dating a cute young girl. Sister upset about school. Cute older neighbor girl fighting with Boyfriend. Best friend who thinks his g/f is going to break up with him. cute female childhood friend. (now we don't know who will be mega)
Then touch thrill of finding the device and realizing it does size-changing. Instead of finding it at school, you take random detour on the forest bike path back from school (senior in HS? 18?), see some charred trees and odd-looking metal, wonder if it's a car accident, pick up this device thinking it looks cool, then run off when some noise scares you away. / Follow some experimentation with the device at home. Disbelief at what it does. Experiment on an some pet/small-animal (hamster?). growth. shrink. More disbelief. / Use it to grow girl in front of you in class. Just a little. Enough to be obvious it did something, and be freaking out. / Make a mistake using it (grow a shoe off someone's foot). Freak out. Realize it can affect sub-parts. / Experiment with subparts at home on 'safe' things. Talk yourself into growing your sisters tits. Grow them a little too much. Freak out. / Next day at school. Grow a few girl's tits. Including a teacher. Get a kick out of it, but realize it's not that much fun, and you can't go much further without it being obvious. / Debate what to do, who to confide in. / overhear sister upset on the phone with bff about how she 'got bigger' overnight, and her b/f keeps trying to feel her up now. cringe at having caused trouble.
Confide in people, setting up the expectation that it will be the path to mega (but pump-fake a few times). Start with the hot-next-door neighbor, setting the expectation you're going to make her hotter and get some. She thinks you're a crazy dork, or high, until she sees it work, then she's freaked out and runs away. Plan fizzled. / Tell the male best friend, who is astonished. Asks a bunch of questions. Wants to use it to get his ex g/f back. / say anything to get her to meet. Show her a demonstration of the device. Her mind is blown. Give her bigger tits. She asks for them even bigger. They warn her it will be very noticeable. She doesn't care. She ends up stacked. She's very appreciative, but won't get back with her ex because he cheated on her. / Now we're troubled. Using the device is just causing trouble.
Start the first real action: cute female childhood friend drops by unexpected. Shuffle to hide the device, but she sees. She asks what's wrong, pulls out the device, asks what it is. She starts playing keep-away, while tinkering with it, while you're trying to get it back. She shrinks something in the room, freaks out, then accidentally shrinks you (by half). She freaks out, closes the door, wraps her head around what is happening as you explain, teases you a little bit. asks if this is what you were upset about, and why. you explain what happened to your sister, and the friend's g/f. She "understands", gives you a funny half size person hug. Asks you to explain how she can grow you back to size. When you ask for the device, she asks if you can even use it on yourself (which you don't know). You explain to her how to use it. She asks how the sub-part targetting works, so she doesn't accidentally use that on you. She sets some settings and instead makes her tits notably bigger. You're upset, but she says to calm down, that she'll get you back to normal soon enough. "I wouldn't be a friend if I didn't let a boy you're age have a little fun with tits this big", in a casual 'friends with benefits' sort of way. with him half-size, her tits are each bigger than your head. (maybe she makes you half-smaller again, for fun)
Start the surprise twist: You pull your face out of a mountainous tit to see your sister in the room, having grabbed the device. Your sister explains that she saw the whole thing, and starts tells you two not to move or she'll shrink you both, and starts ranting on about how crazy this is, that you're a tiny (1/2 or 1/4 size). That now she knows how her tits got bigger overnight, you did it, didn't you. She is fiddling with the device, shoots it at something, shrinking it. Then growing it back a little too big. You try to get her to stop, telling her you need to get back to normal before she breaks the device. She says not to worry, that she'll give it back after she has some fun. She shoots it at herself, and you watch her tits get bigger, until you realize it's actually her whole body getting bigger. She's raving on about how awesome it is. She does it again. Your friend tries to talk her down and reach for the device, but your sister's head is now at the 9' ceiling. She's huge. She realizes she's too big to get out the door, and starts to fiddle with the device with her huge fingers. Suddenly she's activated it, and she's growing even bigger, 'ohh shit'. Her head bumps, then crunches into the ceiling. she tries to sit down, but by the time her butt hits the floor, her head is at the ceiling again. Her hand loses the device, which your friend deftly snags just as your sister falls backwards through the second story outside wall of your house. your sister's leg appears in the jagged opening, until she kneels down, and peers in, the opening hardly large enough for her eye to peer in. She's talking about how surreal this is, as her monstrous fingers enter the opening and tear off a ten-foot section of the wall, causing the roof to sag. Through it she gloats that he wasn't thinking big enough. She sees your friend fiddling with the device and aiming it at her, and she presses down on the edge of the roof, sending fragments of wood, drywall, and insulation crashing down.
How to stop her: when the dust settles, you are held tightly underneath your friend's belly and tits, as she turned over to protect you from falling debris. She has some cuts and scratches, but isn't hurt. Fortunately, she also has the device. 'first lets get you back to normal size', which you're thrilled to hear. She twiddles some settings, and you quickly fill the bed the way you're supposed to. The damage to your room, and the house, doesn't look any less severe. You assert 'we have to stop her'. Your friend has an idea how, but you'll need to go outside. 'I'll tell you on the way. you're going to like it'.
outside: you survey the damage to the lawn, and your house, as the path your sister took away from there / 'let me get this straight. We're going to make you huge like her. You're going to follow her while carrying me. And I'm going to shrink her with the device. and I'm going to like this plan?' . 'yup. pretty simple plan actually. the sooner you get started....' / you are not sure why you don't take much convincing, until you're watching your friend grow in front of you. She has a smirk on her face as first she's taller than you, then her shoulders rise above your head, then her chest, then her waist, then finally the growth stops with the top of her knee just above your head. Her butt is higher than the roof of your house, and it looks like if she sat down, she'd smash it to pieces. She drops to her knees in front of you, setting a hand on the ground which is bigger than a king size bed. 'Ohh, I see what your sister was talking about, this is fucking amazing.' She reaches over to a car, slips her fingers under it, and lifts one side seven feet off the ground, the metal groaning and creaking; then she sets it down with a violent crash. her fingers leaving indentations in the bottom of the side-panels. 'holy shit. we need to find your sister. she could do some serious damage. Ready for the part you're going to like? now don't drop that device.' / It wasn't really a question as she tears the top of her shirt open, plucks you up with her thumb and forefinger, and wedges you into her bra, your arms hanging over the edge of the fabric. 'you good in there?' you're completely terrified, looking over thirty-feet straight to the ground, but you squeak out 'i guess so. be careful'.
the chase: as your friend starts to move, her tits jostle much more than you expected, and you feel more comfortable putting the device in your pocket, sinking lower, and holding the edge of the fabric in your hands. All tasks which are complicated by the fact that a titanic breast is bearing down on you, as it rises and falls several feet each walking step she takes. You can see she's following your sister's debris trail, mostly her footsteps having ruined lawns, or her having kicked over a fence or a set of electrical wires. 'ohh shit', your friend says, and you see why. A house with red-brick and white siding is torn open. The roof has been peeled back, splintered wood lying in the yard and all over the second-floor, which you can see right down into. The garage door is also crumpled into pieces on the driveway, where a car is lying mangled. The back of the car crunched like it was squeezed, the roof peeled open, and two deep indentations in the front hood. I wonder if this is where my sister's boyfriend lives, and why she would rampage through his house like this. / My friend, however, is already moving away, still on her trail. It's only now that I notice that several hundred feet away in every direction, people are standing in the streets watching, filming us with their cellphones. Well, really filming my friend, as they likely can't even see you. "looks like we have an audience," i tell her. If she hears you, she doesn't respond. / you reach a major road, and cars are jackknifed in both directions, obvious signs that your sister actually headed up the road. Your friend follows her, trying not to damage any cars or people. Though it looks like your sister was not so courteous. Sporadically cars have corners smashed, or are half overturned. Finally you realize where she's headed. Your high-school. Why, of all places, would she go to your high-school.
Caught up: As soon as we see the high-school, we see her. She's standing over a gymnasium that's less than waist high, throwing a section of the roof towards the soccer field. Your friend speaks up, "you better get that device ready. She might try to run." You dig a hand down past the pressure of her breast flesh to find the device. Then you bring it up close to the edge of her bra, afraid to hang it over the edge for fear of dropping it. Closer to the school, you can see she tore through the tops of many of the classrooms, and a few of the hallways. You are close enough to hear her ranting now. "You think you can fucking fail me. I'll show you what it means to fail. This building is fucking failing to handle me. This roof is failing to handle me. Watch this." She kicked her foot right through the brick side of the gymnasium, pulling down most of the wall, sending chunks flying everywhere. Luckily school was not in session, though you can see a few bystanders several hundred feet away. They think they are at a safe distance. They are probably not. / your friend calls to your sister. asks her to calm down. asks her to stop. Your sister fumes back in rage, that she just thought she could get big and come after her. Well, it's too late. She already got her revenge. Her revenge on the school. Her revenge on her boyfriend. She looked at her hand, and practically collapsed onto her ass, crying. "I'm sorry dylan, i'm sorry." your sister looks up. "i didn't mean to. I was just trying to talk to him, but he kept running away. Then I grabbed him and before I knew it..." she put her head in her hands, bawling. / your friend whispers that 'any time now would be good', and you remember you're supposed to shrink your sister. You fish the device out into open air, careful not to drop it, check the settings, and shoot it at your sister. In just moments, her body starts shrinking down to normal.
Escape: It's only then that you realize there are police and fire vehicles arriving at the school, a helicopter overhead, and some kind of megaphone yelling only half-intelligible commands. You're not sure who they are yelling at. Your friend tells you to put that device somewhere safe, and hold on. You tuck it back into your pocket, tuck yourself down against her breast, and hold on for dear life as she jogs away from the school, down a mostly empty road. She quickly reaches a large wooded park a couple miles away from the school. The trees are finally tall enough to provide some cover for her, and it looks mostly empty. "See, my plan totally worked. Only two things left to do." She sat in a medium size clearing, taking up a significant portion of it. You know you need to shrink her back to normal. You yell up to her, "okay, I'll bite, what's the other thing?" Her hands hug under her breasts, and you feel yourself suddenly lifted five to ten feet in the air. "I wouldn't be a friend if I didn't let a boy you're age have a little fun with tits this big." (mirror wording from earlier)