Mikado High School trains its students as elite mercenary bodyguards and ranks them by skill. Morito Hayama, an unranked escort trainee, is abruptly assigned to protect Kotonoha Nanase, a charge so dangerous that every bodyguard before him has died. Posing as an ordinary student at her side, Morito hides that he is in fact one of the world's few Master Rank mercenaries, and the gap between his paper rank and his real ability drives the early chapters.