During the day, while taking a walk downtown, Kang Soo-Ah Kang Ye-won is forcibly taken away and confined to a psychiatric hospital run by the sadistic director Jang Hyung-sik Choi Jin-ho. The staff refuse to explain why she was brought there, and she is subjected to what they call "medication" that often comes with intimidation and physical harm if she resists. In spite of her fear, Soo-Ah secretly keeps a detailed journal that documents the hospital's brutal acts. After a failed escape, she endures brutal torture as punishment. An orderly, Dong-sik [Lee Hak-ju], grows fed up with the inhumane treatment and contacts Soo-Ah's boyfriend, Woo-jin [Yoo Gun], to arrange a rescue. The attempt goes awry; the trio is captured and Woo-jin is subjected to disembowelment. Meanwhile, Mi-ro [Cheon Min-hui], one of the patients and Hyung-sik's sex slave, accidentally sets Hyung-sik's office on fire. Hyung-sik rushes to recover his safe containing illicit money. Mi-ro helps the trio escape, but Dong-sik stays behind to rescue the others. As Soo-Ah and Woo-jin flee, Hyung-sik attacks and drags Woo-jin away, and both are burned to death. Soo-Ah escapes the facility, returns home, and discovers her abusive stepfather dead from a gunshot. She is later arrested on suspicion of murder and arson at the hospital.
One year later, Na Nam-Soo Lee Sang-yoon works as a programming director at the same broadcasting station, having been suspended the previous year for exposing the corrupt national assembly member. He is assigned to a project investigating what happened to the hospital, which had recently burned down. He finds Soo-Ah's notebook, but there is nothing about the fire; he seeks Soo-Ah, who is now imprisoned as a murder suspect and uncooperative. While filming at the burned hospital, Nam-Soo encounters a heavily burned Dong-sik [Lee Hak-ju]. Nam-Soo questions Dong-sik about Soo-Ah's diary, prompting a tense confrontation. Digging deeper, he uncovers that the hospital was founded by Police Commissioner Kang Byung-joo [Ji Dae-han], Soo-Ah's stepfather, and that Jang Hyung-sik was his protégé. He also discovers the hospital functions as a front for an organ-trafficking and disappearance scheme, where unscrupulous individuals can pay to make enemies disappear. Nam-Soo concludes that Byung-joo manipulated Soo-Ah's mother into surrendering assets and imprisoned Soo-Ah to close loose ends.
Unable to coax Soo-Ah into confessing due to her distress and with Dong-sik seemingly escaping, Nam-Soo broadcasts the truth to spark public outrage and to pressure Soo-Ah to speak. After a massive public reaction, Soo-Ah comes forward, confessing her ordeals and her connection to Kang Byung-joo, while Nam-Soo exposes the incriminating documents against him. For his demise, Nam-Soo theorizes Byung-joo killed himself after the hospital's destruction to avoid a life sentence. Ultimately, Soo-Ah is vindicated in court.
On the drive home, Soo-Ah reveals a twist: she was not the one sent to the psychiatric hospital at all, but it was her late mother. The diary Nam-Soo holds actually belongs to Dong-sik, and the person who joined the rescue in the hospital was Soo-Ah, while Woo-Jin was actually Soo-Ah's late father. The one Hyung-sik killed that day was Soo-Ah's mother. Finally, Soo-Ah admits she killed Byung-joo out of revenge, and Nam-Soo, moved by her story, decides to bury the secrets so Soo-Ah can have a normal life.
