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.

I never meant to fall in love. I never meant to have children. I never meant to become involved in a war. I never meant to lose people I love. I never meant to let us be thrown down here. I never meant to become the royal scientist. I never meant to do lots of things in life. But I always meant to live.

It has been two months since the Monster Kingdom was released to the Human World. The humans are showing slow signs of acceptance but most importantly, Frisk has been accepted into a new family with Toriel and Asgore. (These are my story events and this is my Fanfiction. None of this is canon.)

Sans x Frisk the last experiment no 2398 failed.. they create a monster but... is that he the true monster or somebody else?