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.

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.