BASICALLY, Deltarune goes like this. You are a lonely kid in a small town. Your name is Kris. You have a sweet but slightly overbearing mother, a high-achieving older brother away at college, and a dad who no longer sleeps in your house. You turn up late to school one morning and find that everybody else is already partnered up for the big project. One by one you go around the room trying to persuade your fellow students to take you on. Then... Suzie kicks down the door.
Everybody is scared of Suzie, including your hapless teacher, who pairs you up as a way to mollify this hulking, mean-spirited lizard girl (everyone is a monster in this story, except for you). The teacher sends you and Suzie out on some half-assed errand to get the troublemaker out of the classroom. Out in the corridor Suzie briefly considers eating your face, but decides against it. Then two of you head on into the store room and through a wormhole into another reality.
There's a kingdom in mortal danger down here. A kingdom awaiting destined heroes who, wouldn't you know it, look just like you and Susie. Capes and weapons appear in a burst of light. A slamming overworld theme kicks in. The adventure is on!