Takumi Sumino is a totally average teenager living in the Tokyo Residential Complex, a place where every day is much like the last and nothing bad ever happens. All that changes when freakish monsters attack the town and start wreaking havoc. A strange creature calling himself Sirei appears and offers Takumi the power to protect those he holds dear... All he has to do is stab himself in the chest!

The next thing he knows, Takumi is in Last Defense Academy, a school in the middle of nowhere surrounded by a wall of otherworldly flames. He and 14 other students have been drafted into the Special Defense Unit, a team tasked with keeping the school safe for the next 100 days. How much are they willing to sacrifice to take back their normal lives and save the world from the grotesque school invaders? The curtain rises on 100 days of war and despair...