In a world where every corner has been explored, the only remaining secret is the large cave "Abyss". The huge vertical cave, which is bottomless and extends to an unknown place, is inhabited by strange creatures and contains precious relics that cannot be made by modern humans. The incredible posture of the "Abyss" attracts people and drives them to adventure. In this way, adventurers who have challenged the large cave many times have successively been named "cave explorers". Riko, an orphan living in the town "Oss" built on the edge of the abyss, dreams of becoming a great cave explorer like her mother one day and solving the mystery of the abyss. One day, while exploring the abyss, Riko picked up a robot with the appearance of a boy...