In the year 298 of the Divine Era, elementary school sixth graders Washio Sumi, Minowa Gin, and Nogi Sonoko are appointed as "Heroes." The sole duty of the Heroes is to protect humanity from the attacks of the monstrous enemies known as Vertex, who descend from the heavens. Initially, the three are disjointed, but they gradually begin to coordinate their efforts and successfully repel the Vertex. However, new Vertex continue to appear one after another, and one by one, they begin to suffer injuries――.