Kai Setsuna moves to a certain town to live alone due to family circumstances. However, shortly after arriving, a mysterious rain of stones falls on the town, and the apartment room he was supposed to live in is crushed by a massive boulder. As Setsuna is at a loss, he encounters Kaname Hiroumi, who is collecting data on Makai. Hiroumi informs Setsuna that alongside the real world, there exists another world called Makai; events there affect the real world; and that Setsuna is a Devil Child destined to fight the Great Demon King Lucifer, who plots to rule Makai. Led by Hiroumi to the research institute, Setsuna meets the other Devil Child, Kaname Mirai, and her partner, Bel, and heads to Makai. 　　Thus begins Setsuna and Mirai's battle to protect the peace of Makai.