Frisk and her sister Chara are out walking on Mt. Ebott; the place of the urban legend of the Underground, a place where monsters, who were cast away by humans, live; when suddenly, Frisk takes a fall into the realm of monsters. She encounters many monsters, a few that really warm her heart, but when she goes through some rough patches, even the most special monster to her, Gaster Sans, one of the skeletal brothers, isn't able to help her. What will become of Frisk and Gaster Sans' relationships?