Asuna who has been forced to grow up quickly ever since her father pass away, while her mother, a nurse, works long shifts at a hospital. Asuna spends her solitary days listening to the mysterious music emanating from the cat's-whisker aka crystal detector receiver her father gave to her as a memento, accompanied by pet cat Mimi. One day, a strange creature appears on the surface and nearly kills Asuna. Luckily, a mysterious young man named Shun saves her. From that encounter, Asuna finally learns of the existence of the underworld called "Agartha." However, when Shun suddenly disappears, she ventures out to investigate the underworld. Together with her teacher, Morisaki, Asuna begins her journey to explore "Agartha" and discover the truth behind Shun's disappearance and her own past.