Yonosuke Hikura appears as an ordinary high school student who unexpectedly inherits his family's generational duty: safeguarding the harmony between Heaven and Earth. He wields the magical sword Chitentai, a weapon capable of slaying demons that also acts as a gateway between these realms. Despite this immense responsibility, Yonosuke continues his school routine, though he is frequently late and displays a spoiled attitude, reflected in servants addressing him as "Yo-dono" at home.

His companion is Tsukinojo Inbe, a protector dispatched by the high Priests of Earth. Tsukinojo aids Yonosuke in fighting demons escaping from the "Earth World," a distinct realm separate from the human world. Confusingly, "Heaven" refers to Earth itself within their context. Tsukinojo possesses supernatural powers and can shrink into a chibi-like form, residing within a talisman Yonosuke carries. The central plot involves Yonosuke losing this talisman, triggering confrontations with shadowy organizations and demons aiming to invade Earth.

Yonosuke relies heavily on his partnership with Tsukinojo during their battles against demonic forces. His initial portrayal emphasizes tardiness and a privileged upbringing as key flaws. The narrative concludes with him facing school punishment despite his actions to stop the demonic invasion, highlighting the unresolved nature of his personal journey within the OVA's runtime.