(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"!
~'Disclaimer: This story has a lot of profanity and covers some very very intense or touchy subjects that may disturb some readers. You have been warned.'~
((Unedited))
After Chara fell into the Underground, they met Asriel a monster inhabitant of the Underground. Soon after that, Chara joins a new family, but their ambition to keep everyone happy goes a little overboard and Chara is flung into a reality of destruction and hopeless wonder. Then many long years later they meet the beloved Frisk, who fell into the Underground, similar to themselves. Eventually everything Chara thought they knew correctly is turned upside down and they as a person are changed forever.
~'The game Undertale belongs to Toby Fox and some of the dialogue is ripped straight from the game itself, too.'~