Last Days and Birthdays

(First Undertale fan fiction, so it'll suck. Sorry, but hopefully you don't have too bad a time reading this. As I add on, it might contain fluff, but no lemon smut, so this isn't that kind of fan fiction, sorry)

Frisk is now 18. It's their birthday, and their last day of high school ever. Everything looks great, except everyone goes back in time. Someone is resetting the timeline, and it's up to Frisk, with the help of their boyfriend Sans, to make it right, or the biggest danger will be released: Chara. Can they do it? Or will Chara come out to play? Find out in "*Stay Determined"!