Early 20th century. Polina arrives at a girls' boarding school on a mysterious island to find her sister who disappeared under strange circumstances. Polina's investigation is complicated not only by conflicts with other girls and the imperious mentor Zakharova but also by a strange obsession that drives the girls mad, one after another. Polina needs to find out what happened to her sister without losing her mind in the process and get off the island which is sinking in the abyss of insanity.