[4][5][6] Set in Gyeongseong in 1938 during the Japanese occupation, the film centers on Ju-ran/ Shizuko (Park Bo-young), a sickly young girl who is transferred to a sanatorium/girls' boarding school to recover her health.

When the headmistress and the soldier leave the office, Ju-ran and Yeon-deok continue their snooping, eventually finding an experiment proposal written in Japanese.

It describes a performance-enhancing drug that aims to alter the human condition, making the subjects super strong but also heightening their emotions and causing many adverse side-effects.

This reveals that Imperial Japan is using the girls as lab rats to create drugs to enhance performance of Japanese troops against the Allies.

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.

Ju-ran, bleeding badly and distraught at her friend's demise, throws the principal at a spike hanging on the wall, killing her.