Naraka High School lies above the clouds on a floating island over the bustling streets of the Saitama district. Graduating from this prestigious academy can improve one's social status, and anyone who is able to see the school after applying has been accepted as a student. Such is the case for Momiji Rokudou, who views this as his big chance for a fresh start and a new purpose.

Upon arrival, students are asked to write down a word of their choice that they will use later on. On his right arm, Momiji writes the kanji for "change." Unfortunately, the change he awaits becomes a nightmare when the school is flooded with monsters, killing those unfortunate enough to get in their way. Momiji and his fellow classmates must now survive the neverending hordes of beasts in order to graduate from this hellish institution; who said high school was going to be easy?