Kasumi is one of 160 people infected with the Medusa virus, a plague that turns the body to stone, chosen for cryogenic sleep aboard a sealed facility called the Thorn while a cure is sought. She enters cold sleep without her twin sister Shizuku, and when she wakes the place is overgrown with thorns and roamed by monsters, with only a handful of other survivors and no clear answers about what happened while they slept.