frisk doesn't really know what happened, one moment she's on her journey to get out of the underground, the next everything has lost its happiness, her friends gone. not sure what to do, she uses her determination to try to get to then end but maybe two brother figure out the plan, putting aside their dreams to tease and slow down frisk from her task. undertale has changed to underfell or as frisk calls it; underhell.
5 parts