Five years ago, something happened to Mari Wakatake. Something that took the lives of every other human on Kamioki Island, then wiped her mind clean of even memories. But if Mari's past is an unknown nightmare, her future may soon become even more terrifying. Because while Mari was the sole human survivor, something else lived through the same night, and now, wearing a female skin, its path is about to cross Mari's again at the exclusive Kaihou Academy.