In the Undertale universe, a different human child falls down into the underground. Even though she is very young she will have to find her way home or stay there forever. ----- Big thanks to my friend from school who helped me out a bit.

After Frisk goes missing, Caitlin, Frisk's twin sister goes looking for her in the the last place she saw her. MT Ebott, the mountain of dark legends, she finds a necklace that belonged to Frisk on the ground. Just like Frisk, Caitlin tripped and fell into the Underground. Caitlin finds things she never knew existed.

"Promise me one thing- just one thing only." Frisk nodded vigorously. "No one can know about this whole reset thing. Not Papyrus, not Asgore, not anyone. It would basically bring this world to the point of falling to pieces at our feet. If we weren't trapped between time and space already, very few of us wouldn't be able to handle that knowledge, and could drive those who couldn't to the edge. So, please. I'm begging you. When you reach the End, when you get us out of here for the umpteenth time, I don't care if you reset or not. Just don't tell anyone about it, and... Please don't go homicidal again, okay?" Frisk smiled meekly, and nodded. "I can do that. And don't worry about the reset." They gave Sans a small hug. "After that whole thing with Chara, there won't be any more." And with the final word, they opened the door to leave; before exiting they paused. "See you later."