Yuta Hibiki, a freshman in high school living in Tsutsujidai, wakes up one day with no memory. Later, Yuta meets "HyperAgent Gridt" that appears in an old computer. Gridt tells Yuta to "complete the mission", and Yuta begins to search for the meaning of these words and his own memories. Although confused by this sudden incident, Yuta still lives every day with the help of his classmates Utsumi Masaru, Takarada Rikka, Shinjo Akane and others. However, the peaceful daily life is easily crushed by the sudden appearance of a monster.