Sans has lived through countless timelines. He's seen the kid make friends with everyone, only to give up in the final moments. He's seen the kid killed, over and over and over again. Sometimes, he'd even done the killing himself. And no one ever remembers. Not even the kid herself. It's an exhausting cycle, one that never seems to end. But there's something different about this run. Another human's fallen down... the kid's sister.

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.