Shinpei Ajiro resided with the Kofune sisters, Mio and Ushio, when his parents died, but he now lives alone in Tokyo. Ushio, on the other hand, comes home to lament the loss of Shiori Kobayakawa when she drowns while attempting to save her. However, marks around Ushio's neck cast doubt on her cause of death. When Shinpei takes a step back, he realizes there must be another explanation. Dangerous monsters prowl the islanders, and a vision of Ushio, who has passed away, urges him to "rescue Mio" as her dying wish. He's convinced that something is going on, and that Ushio's death is just one piece of the jigsaw. Shinpei, with the support of Mio and others, tries to carve out a future for himself and his companions.