Yuuta Hibiki is a first-year student in high school living in Tsutsujidai. One day, he wakes up without his memories. Yuuta later meets Hyper Agent GRIDMAN who is inside an old computer. GRIDMAN tells him to fulfill his purpose. Yuuta's quest to understand the meaning behind those words and to find his memories begins. While all of this is sudden, Yuuta has helpful assistance from his classmates, Shou Utsumi, Rikka Takarada, and Akane Shinjou, but...

Those peaceful days are destroyed mercilessly by a monster that suddenly appears.