Kokushi no Shima

A few days ago, non-fiction writer Katsuragi Shiho came to see Shikibu Takeru at his office in Tokyo. She gave him the key to his apartment and asked him to take care of her things if she did not return from her trip in 3 days. Unfortunately, she did not return in 3 days, nor in 4. Even more mysteriously, nobody seemed to know where she had even gone. Worried, Takeru dug deeper and eventually found 2 witnesses that said they saw Shiho and another woman who looked her age ride the ferry to "Island Kokushi" near Kumamoto. Takeru sets off to try to find her there, but what awaits him is a strange island where nothing is what it seems. In a place haunted with old tradition, a strange religion, and several mysterious deaths, can Takeru find what really happened to his friend?