Yami no Koe

A group of six people--two teachers, two male and two female students--rents a yacht, only to find a stowaway, a girl named Tachikawa Nanami. Shortly after, violent seas wreck their ship and force them ashore an island. What are the intentions of the beautiful woman named Sayoko and her maid K, who offer them shelter in her mansion until the ferry arrives in 30 days? What are Nanami's intentions? Time is running out.