(I know this AU idea has probably been done to death by now, but it's genuinely my favourite concept in this fandom and I've always been obsessed with making my own version of it 🙏)
In a nutshell, the weird-ass space goo Pim injects into his arm during the party ends up fucking with his DNA as a Critter later on, causing him to start developing alien traits because of it (Initially just stuff he could easily hide, such as pair of antennae and slightly green-ish pupils, but it later develops further into psychological traits as well, such as nocturnality and an instinctual longing to return to space)
Despite his usual apathy, Charlie finds out pretty quickly that Pim's hiding something (as he would, considering that they're together on jobs nearly every day), and once Pim tells him the truth, he opts to help him hide these changes for as long as they can (such as lending Pim his hoodie to hide his most prominent alien features, making up sick-day excuses for him, ect...), or at least until they can find a cure of some kind, despite not knowing where to start looking.
They both know they can't tell anyone about these changes happening to Pim, fearing that it might all end up in one of them accidentally blurting out what happened during that Saturday night that all culminated in Bill's death and them finding out that the earth is flat (both of which they're still trying to recover from, on top of all this.) --- I have a whole bunch of different ideas for this au that may not even make it into the final product (Glep was initially going to find out about Pim as well, but I scrapped that idea in the end, seeming as it kind of took away from the whole tension of P & C having to keep the UFO Hunting incident a secret), so maybe I'll make another, more refined post about this AU someday?? Who knows??? Bonus old doodles back when this AU was still in its baby stage: