The backstory:

A bunch of kids in the high school anime club decide to throw a birthday party for the little brother of one of the members:

• Hallie Mathews, a cat-ear-headband-wearing card game nerd with definitely the normal number of parents (our character!)
• Samantha "Sayaka" Dross, the self-proclaimed Eternal Knight
• Evan "Ikei" de Lune, a foreign exchange student from Japan
• Lillia Hartwell, Sayaka's sidekick
• Rose Rowell, approximately the only sane one
• Isaac, the one with the little brother
• James, the little brother
• Mary, who the GM compared to "Rena Ryuugu but" and the second half of that comparison is a spoiler 😁

They spend the prologue session grabbing the cake (two cakes, actually!) from a shopkeeper named Tony Spiders, nyooming around on heelies, and generally being very silly so everyone lets their guard down. (It's a horror campaign "inspired by Silent Hill, Corpse Party, and Weaver's Nan Quest series". This silliness isn't going to last.)

When they get back to school to set up the party, they somehow get thrown into an evil parallel version of the school, James is missing, Mary is talking nonsense, and there's this twisted monster chasing them around.