From Girls' Generation Scanlations:

Katsuragi Shiho casually handed her house key to Shikibu before leaving her hometown. After not returning on the third day as she had promised, Shikibu probes around and finds a lead that brings him to "Yasha Island".

However, the strange rituals being held on the island, Shikibu's investigation records getting stolen, and finding Katsuragi's name in the house of blood among other mysterious occurences all point to Katsuragi having vanished into thin air! Shikibu is bent on finding out the truth, but the islanders all seem to have been silenced by somebody...?!