Okay so it first starts off with a group of scientists working on a top secret project. I don't really remember what it was or if I even decided on what it was, but it probably had something to do with the mind and psyche. The name of the game is an acronym and the names of these scientists made up the letters of that acronym which was a sorta big reveal. Anyway, the thing that these scientists were working on goes awry, as all top secret secret "science" projects do. And what exactly does this cause? It causes people exposed to it to turn into their favorite fictional character. (This was my way of explaining how all of these random characters are all together in this world.) I also think that if someone else was already turned into your favorite character, you'd either turn into an alternate version of them, as a way of explaining alts, or you'd turn into your second favorite character and that would go on until you eventually turn into something that someone else hasn't already been changed into.
At first this only effects the lead scientist's son, who turns into Blu from the Rio movies. Then I think then they might mercy kill him for some reason. But eventually, the project gets even more out of control and effects either the entire world with a few remnants of unturned humans left or just the project's home country of Brazil with a couple of unturned humans left...
...Then the scientist's son who was turned into Blu from the Rio movies comes back as a super zombie bird thing with a bunch of powers and takes over the entirety of Brazil. He then rules over the country with a zombified iron wing and it's up to the fighters of the game and the few unturned people left to take him down. This is what the game's classic or story mode was about. Also all of this backstory wasn't like, immediately thrust upon you when you started the game. Instead, it was revealed to you through the standard video game style logs that you'd find by playing. Anyway, at the end of the classic/story mode, you fight superpowered zombie Blu on the top of the big statue in Rio (that acts as a FD clone) and eventually you would've beat him and won. So now he doesn't rule Brazil anymore. Also the whole people getting changed into other things wasn't really dealt with at all. The characters are all just like "well, guess we have to live with knowing this horrifying secret about the nature of our existence now." and they just kinda chill. You'd expect it to end with everything being changed back or something but nah.
Now you may be asking "Baysha, why did you lean so hard into all the Brazil stuff?" Was it because I'm Brazilian? Nope, not Brazilian at all. Did I really really like the Rio movies? Nah, I thought they were okay but that's about it. The real reason was...
...so I could have each reveal trailer be a "you are going to Brazil!" meme and then I just kinda built off of that idea.
side note: it's pronounced "gener-ee pree", not "generics pree".