Set against the backdrop of Gyeongseong in 1938, during the harrowing period of Japanese occupation, this film follows the journey of Ju-ran/Shizuko, portrayed by Park Bo-young, a fragile young girl who is relocated to a sanatorium that also serves as a girls' boarding school. This transition sets the stage for her to regain her health, and soon, Ju-ran finds solace in the companionship of her new friend Yeon-deok, beautifully depicted by Claudia Michelsen, and benefits from a specialized treatment program orchestrated by the headmistress.
However, as Ju-ran begins to recuperate, she becomes increasingly aware that students are mysteriously disappearing from the school. In addition to this unsettling phenomenon, she starts experiencing abnormal changes in her own body that leave her perplexed and alarmed. Driven by a relentless curiosity, Ju-ran embarks on a quest to uncover the unsettling truths hidden within the school’s walls.
As her investigation unfolds, Ju-ran witnesses her classmates in horrifying conditions, some bleeding out, while others suffer violent seizures. The school's administration, however, dismisses her concerns outright, attempting to convince her that what she perceives is merely a figment of her imagination. One incident, particularly revealing, involves her classmate Yuka, played by Jule B, which leads Ju-ran to a violent outburst, showcasing extraordinary strength that leaves her confused about her newfound abilities.
Seeking answers, Ju-ran and Yeon-deok resolve to break into the headmistress' office. Their efforts yield shocking revelations when they overhear a heated exchange between the headmistress and a young man wearing a Japanese military uniform. This conversation reveals dark intentions centered around a new drug and the troubling fate of the disappearing students. As they continue their search, the duo uncovers an experiment proposal written in Japanese, detailing plans for a performance-enhancing drug intended to alter human capabilities, but with horrific side effects.
This discovery unveils the grim reality that the girls are being used as unwitting test subjects for the Imperial Japanese Army’s ambitions. Their horror deepens as they find footage capturing previous trials on their cohort, which illustrates the terrifying effects of the drug. Their investigation spirals into chaos when they hear a commotion outside leading to an attempted suicide by Yuka, prompting them to flee into the forest. However, their escape leads them to the edge of a massive Japanese military base, resulting in their capture.
The commanding officer asserts control over the experiments, leaving the principal distraught and the entire facility in disarray. Ju-ran and several girls, now sedated and subjected to a new variant of the drug, face an even darker fate—that of potential violent side effects. Meanwhile, Yeon-deok stumbles upon a room filled with tanks of preserved bodies, revealing the grim truth about their vanished classmates. When she is discovered, she meets a tragic end, drowning in one of the tanks, while her cries echo unanswered.
This trauma awakens a primal rage in Ju-ran, who embarks on a vengeful rampage against their captors, killing numerous Japanese soldiers. Gaining the resolve of the other girls, they confront the doctor, resulting in a brutal showdown. The principal’s desperation culminates in confrontation, leading to her wounding several girls, including Ju-ran. As chaos engulfs them, Ju-ran, grief-stricken over Yeon-deok's loss, retaliates and kills the principal in a climactic moment of tragedy and sorrow.
In the final scenes, we find Ju-ran desolate beside Yeon-deok's lifeless body, whispering bittersweet words: “Let's go home now, you and I.” Ju-ran’s journey concludes in a poignant moment, leaving behind a haunting message of survival and the scars of war.
