In a world where every corner has been explored, the only remaining secret is the large cave "Abyss". The huge vertical cave that is bottomless and continues to an unknown place is inhabited by strange creatures and precious relics that cannot be made by modern humans. The mysterious appearance of "Abyss" attracts people and drives them to adventure. In this way, adventurers who have challenged the large cave many times have been named "cave explorers". Riko, an orphan living in the town "Oss" built on the edge of Abyss, dreams of becoming a great cave explorer like her mother one day and solving the mystery of Abyss. One day, Riko decides to dive into the depths of Abyss when her mother Raisa's white flute is discovered. Riko picks up Reg, a robot who has lost his memory, and decides to go with him to find his own memory. Riko was poisoned by the Bullet Piercer on the fourth floor of the Deep Realm. The one who saved Riko was the "living corpse" Nanachi. Nanachi also became their companion, and the three of them set out on an adventure to the fifth layer of the deep world where Podordo was. There, they met a girl who called herself Prushka...