Okay, we're starting in Wilko...
Sorry, but the story can't start quite yet though. We still have to determine some stuff.
Like, is this just possession? Like someone becoming or discovering that they're a body hopper or something. Just hopping from body-to-body, possessing them in a variety of ways. And then later, figuring out how to mess around with multiple bodies or something, mixing bits and pieces from the people they've hopped?
Or, is this a mix and match? Like, maybe multiple people in Wilko (or around, like in Kwik Save, for example) suddenly gain a bunch of strange powers? Like maybe someone gains the ability to switch body parts around, someone can switch more abstract things (like personality, voice, accent, etc.), and someone else can possess others? And maybe some of them don't even realize they have these powers at first, accidentally swapping things around without realizing it at first?
Or maybe you're less interested in someone (or a few people) having powers, and prefer something more technological or magical?
Like, maybe someone discovers an app has auto-installed itself on their phone. And the app has a bunch of really odd features, like allowing the app-user to possess someone and mess around with people's traits. Like, maybe copying/pasting different body parts and such could be an ability of the app. Or maybe it's something like a game with multiple people included? Where you "capture" people with the camera and store them inside, able to do whatever you'd like with them? Which... I imagine includes sexy and kinky things. Maybe it could fuel the inter-store rivalry as well, like there's "competitions" at the end of each day between the stores, that use their captured people? Which, knowing how perverted some people could be... the competitions could be something serious or something less so. Like maybe, the competition each day changes, one day there's a wet t-shirt contest, the next day there's mud wrestling, and so on. Could be fun.
If we're going the magical route, maybe there's some magical, enchanted, or cursed item or items. Some ideas could be that it's a magical item, like a ring or a necklace, that allow its user to take anyone's body parts or traits as long as they're in eyesight. Could be something as small as a mole below someone's eye, or as big as their entire body to use as their own (basically possession). Of course, this would be more magical than grounded in reality. So if someone, for example, "took someone's mind", it wouldn't kill them, it would just make them really susceptible to persuasion, or knocked out until they get another mind in there, or something along those lines. No murder, please. If you'd prefer something cursed, maybe whoever uses it gets possessed by someone inside it or by someone who touches them? And maybe when they're possessed, the possessor can reshape their body how they see fit. Want to give them bigger or smaller boobs? Want to add a few inches to their cock, or add one? Got a thing for redheads? The possessor can do whatever they want. But maybe with a limited time frame, or once per possession, risking losing their host forever if they want to leave and re-possess them and try something new.
These, of course, are just some ideas. If you really wanted to... I guess you could probably do something like aliens, or something? Maybe they take vacations inside earthlings and temporarily (or permanently, if you want, I guess) alter their bodies to suit whatever they prefer in hosts? I... I don't really know.
These are only some options though...