Following the death of his parents, Shinpei Ajiro lived with the Kofune sisters, Mio and Ushio, but has since moved to live alone in Tokyo. However, after Ushio drowns while attempting to save the young Shiori Kobayakawa, he returns home to mourn her passing. Yet, bruises around Ushio's neck cast doubt on the cause of her death. Stepping back, Shinpei becomes convinced there must be another explanation. Dangerous entities roam among the islanders, and a vision of the deceased Ushio asks him to "save Mio" as her final request. He is certain that something is afoot—and Ushio's death is only one piece of the puzzle. With the help of Mio and various others, Shinpei struggles to forge a future in which he, his friends, and family can survive. But the more he investigates, the more impossible such a future seems. This darkness infiltrates his home island. Just how much more suffering will he endure before finding the right path to save it all?