Ever since his mother passed away, first-year student Setsuna Kai has sworn to become stronger for her sake. Despite tireless daily training at his elementary school's kendo club, he still cannot defeat his rival and friend, Mirai Kaname. However, during their everyday walk home from practice, an ominous black haze appears over Tokyo, blocking out the sky and plunging temperatures, covering the city with crystal snow in the middle of summer. As Setsuna returns to his dark apartment, a child flies through the walls and hands him a wrapped box before taking his last breath in Setsuna's arms. A demon, hellbent on taking his life, appears before Setsuna can process what had just occurred. At death's door, Setsuna's conviction to live awakens a power from deep within him, allowing him to summon a demon of his own. The Cerberus demon named Cool explains to Setsuna that he is one of the eight Devil Children who fought Hell and must now fight to save the world from destruction. Though he is vehemently opposed to accepting such an important responsibility, Setsuna's determination to surpass Mirai drives him to assume the position. Alongside Mirai—who also happens to be one of the fated Devil Children—Setsuna dives into the Demon World to locate the other six in order to fulfill their mission.