Set in Gyeongseong in 1938 during the Japanese occupation, the film centers on Ju-ran/ Shizuko (Park Bo-young), a sickly young girl who gets transferred to a sanatorium/girls' boarding school to recover her health. Her physical condition improves thanks to her new friend Yeon-deok, and the headmistress's special treatment program. But she soon notices that students are disappearing one by one and that her own body is undergoing abnormal changes. Determined to uncover the truth, Ju-ran starts to investigate the mysterious happenings and the school's possible role in them.
Shizuko begins seeing a couple of her classmates in terrible positions; bleeding out or having severe seizures. The school denies that anything unusual is happening in an effort to convince Shizuko that it is all in her head. After being confronted by her classmate Yuka, Shizuko snaps and in a violent rage almost strangles Yuka to death with a single hand showing incredible strength. Shizuko does not understand what is happening to her or to the other girls in the school and with the help of Yeon-deok, who now recalls other horrors from her time there, decides to investigate by breaking into the headmistress' office. While in the headmistress' office, they overhear an argument between her and a young man dressed in a Japanese military uniform. The argument is about a new drug and apparently losing test subjects, referring to Shizuko's disappearing classmates. When the headmistress and the soldier leave the office, Shizuko and Yeon-deok continue their snooping eventually finding an experiment proposal written in Japanese describing a drug that aims to alter the human condition making the subjects super strong but also heightening their emotions and causing many adverse side-effects. Shizuko and Yeon-deok also find movies of themselves and a previous cohort that demonstrate the effects of the drug. The two students then hear a commotion outside culminating in the attempted suicide of Yuka. In the ensuing chaos they try to escape the school by running into the forest, only to find that they are at the edge of a massive Japanese military base. The film is a scathing critique of Japanese imperialism and shows the resilience of two young Korean girls that attempt to live free.
