The Hundred Line -last Defense Academy- has Takumi Sumino, a normal teenager who lives in a residential complex in Tokyo where the days spend without penalty or glory. He has no great joys or lives many adventures, but nothing bad happens either. However, Takumi's peace is interrupted when a strange monster attacks the city and begins to sow chaos. At that time a mysterious creature called Sirei suddenly appears and gives our protagonist the power to protect their loved ones, but in exchange for the gift a knife must be stuck in the chest.
After this encounter Takumi appears in Last Defense Academy, where he is registered without knowing very well how, along with 14 other students. The academy is in the middle of nowhere and surrounded by a wall of supernatural flames. Takumi and the rest of his classmates are not long in being informed that they have been recruited by the Special Defense Unit, a team that has the mission of protecting the school during the next 100 days. If they make the sacrifice, young people will not only recover their lives, but also save the world.
When the invaders attack the Academy, a tactical battles system will be launched. During his turn, the player can use a limited amount of skill points to move students by a grid and use their skills. The attacks that are successful fill a meter that once complete allows additional movements or special attacks. In addition, students have a technique called Last Defense that can go around combat, but it can only be used when little health has left.
Outside the fight the students of the Academy have free time to explore the facilities, strengthen ties and complete different activities that allow you to gain points. With experience and other resources weapons can be improved. In addition, in several points of the campaign the player will be forced to make decisions, which will lead to a branched history with different finals.