After many resets, Frisk finally decides to finish with a Pacifist Run and let everyone have their happy ending. With humans and monsters slowly coming together, everything seems to be falling into place and he couldn't be happier. But Frisk remembers something: The other six human souls. Their bodies were left in the Underground without a proper burial, one of which they deserve. However, when he and Toriel go up to retrieve the bodies of the children, they discover something rather unexpected.

Frisk, the final human, has finally fallen into the Underground, her soul ripe for the taking, or so they'd thought. When this human begins committing genocide throughout the Underground, many monsters stand up to fight against her, including a small monster named MK, who gets himself into more trouble than he had anticipated.