Takumi Sumino is a completely ordinary teenager living in a Tokyo Housing Complex, a place where every day is similar to the previous one and nothing bad happens. Everything changes when strange monsters attack the city and start causing chaos. A strange character who calls himself Sirei appears and offers Takumi the power to protect those he loves... All he has to do is shoot himself in the chest! The next thing he knows, Takumi finds himself at the Last Defense Academy, a school located in the middle of nowhere, surrounded by a wall of supernatural flames. He and 14 other students have been selected for the Special Defense Service, a team responsible for keeping the school safe for the next 100 days. How much are they willing to sacrifice to restore their normal lives and save the world from the bizarre school invaders? The curtain rises for 100 days of war and despair...