- The story is centered around an orphan girl named Riko who lives in the town of Orth on an island in the sea of Beoluska. The city surrounds a strange, giant hole going deep into the earth, which is commonly called the Abyss. The Abyss harbors artifacts and remnants of a civilization long gone, and is therefore the popular hunting spot for so-called Cave Raiders, who undertake arduous and dangerous descents into the mist-filled pit to recover whatever relics they can find. However, the deeper one goes into the Abyss, the more they are afflicted by a progressively fatal malady called the Curse of the Abyss once they ascend back; few who have descended into the lower regions have returned to tell of their experiences. Some legendary Cave Raiders earn the title of White Whistle, one of them being Riko's mother Lyza.
Riko's goal is to follow in her mother's footsteps and solve the mysteries of the Abyss. One day she explores the caves and discovers a robot named Reg who resembles a human boy that she and her friends befriend. Some time later, Riko is informed that some items sent by Lyza were recovered, including a message to her, telling her to meet in the bottom of the Abyss. Riko then bids farewell to her friends and departs to the Abyss with Reg in search for her mother, despite knowing of the risks and the fact that due to the Curse of the Abyss, she will never be able to return.