Almost four years marking the anniversary of monster kind reaching the surface, the human ambassador Frisk goes missing. Severing the link between the two species, an all out war breaks between humans and monsters. After bloodshed on both sides, monsters emerge victorious.
Yet somehow a miraculous town in San Francisco has evaded the warfare and even has an orderly society. And despite everything, Sans keeps encountering a girl who has saved his life one too many times.
When your fate and Sans' intertwine, it becomes apparent that the relationship between the two of you extend beyond this timeline. You are plagued with memories of a distant life and Sans begins to hallucinate visions of a not so distant past.
Insanity seems to be eating away at the two of you and it's only a matter of time before someone cracks.