When a body is found in a hot-spring town, dead from hydrogen sulfide poisoning, geochemist Shusuke Aoe is brought in by the police to investigate. The death is written off as an accident, but something doesn't add up, and at the scene, Shusuke comes across a young woman searching for her missing friend. Two months later, he finds himself examining an almost identical incident in a completely different part of Japan. Are the cases related? Did the victims know each other? As these thoughts swirl around his head, Shusuke encounters the same young woman he met at the first crime scene. And before his very eyes, mysterious events begin to happen around her...