You were out at a new bar drinking alone when you find a yellow jacket with green sleeves and a hood. You put it on in your drunken state. You eventually black out.

You wake up with a hangover in the trunk of a car the next morning. The trunk is wide open and there's a beer bottle inside with you. You realize you feel weird. You realize you're a girl and you're Chara. You feel your phone ring in your pocket, you realize it's Frisk, you somehow recognize that name in your new body. You are stuck as Chara forever now.....