When the Black Sun and its Breakdown Phenomenon destroyed Atara Kasuga’s hometown, his cousin Hiijiri managed to put a magic grimoire around his neck for protection just before she and everything else were swept away. Rescued from a false fantasy world by the magus Lilith, Arata soon discovers that Hiijiri is still alive and that the amulet has given him access to great powers. However, before he can rescue his beloved cousin, he must first enroll in Royal Biblia Academy, face seven beautiful female mages whose abilities match the seven sins, and learn how to use his powers to become the ultimate demon lord.