Jin-seok [Kang Ha-neul] moves into a new home with his mother [An Min-young] and his father, and his older brother Yoo-seok [Kim Moo-yul]. The father warns that one room contains the belongings of the previous owner and that they must not enter it. That night, as Yoo-seok takes Jin-seok outside in the rain for some fresh air, Yoo-seok is suddenly kidnapped and dragged into a car. After 19 days, Yoo-seok returns home, but with no memory of his disappearance.
Jin-seok begins to notice unsettling discrepancies in his brother’s behavior and grows convinced that Yoo-seok isn’t truly his brother. Yoo-seok attributes these oddities to missed daily barbital medication. As Jin-seok pieces together signs of deceit from his parents, he makes the bold decision to flee the house. He seeks help at a police station, but the officers doubt his account, implying that he might not be sane. In a shocking twist, the officers reveal the year as 2017, not 1997: Jin-seok is actually 41, and the world around him is hooked up to modern technology. He confirms this by noticing a smartphone, a tablet, and an LED TV displaying Moon Jae-in, the president of South Korea, meeting with Donald Trump on screen Moon Jae-in Donald Trump.
Returning home, Jin-seok dares to enter the forbidden room and stares at a stark depiction of a murder scene. His family soon arrives, and Yoo-seok finally drops the veil of secrecy, revealing a hidden history that had haunted them all. Twenty years earlier, a teenage girl and her mother were killed in that same room. The cold case lingers, and the victims’ family begins to hunt for the killer, recruiting Yoo-seok in the process. They abduct Jin-seok, convinced he is the killer, and subject him to torture until he claims amnesia or recollection of the event.
A psychiatrist is brought in to help Jin-seok recover memories, suggesting dissociative amnesia as the explanation. The plan involves replaying the events of 1997 in a controlled reenactment: the psychiatrist would play the father, while Yoo-seok hires an actress to portray the mother. The mother, for her part, refers to Yoo-seok by his real name during the ruse. The restricted room is used for the reconstruction, with the intent to complete the confession on a stormy night. But the plan falters when the real Choi—who with his arrest for assault and fraud complicates the ruse—remains a destabilizing obstacle. The group must press on and keep up appearances while waiting for the next rainy night to finish the reenactment.
In the present, Jin-seok remains convinced of his innocence. He escapes the clutches of Choi, who pursues him in a van. A crash injures Choi, and Jin-seok is struck by a passing car, waking in a hospital with memories from 1997 returning in full force.
The 1997 sequence unfolds at a river, where Jin-seok’s real parents and Yoo-seok watch from a car before a fatal crash claims the parents and gravely wounds his brother. Financial pressures during the Asian financial crisis complicate Jin-seok’s ability to arrange Yoo-seok’s surgery. An anonymous message offers Jin-seok money to kill a mother, on the condition that he spares her two children. Driven by urgent appeals from Yoo-seok’s doctor, Jin-seok reluctantly accepts. Yet when he confronts the mother, his conscience fights back. The mother awakens and mistakes him for her husband; a struggle ensues as the daughter wakes, screams, and runs toward her room. Jin-seok attempts to calm her, and in the tense moment, he ends up stabbing the girl while struggling to escape through the door. The father, revealed to be the doctor, arrives and screams, and Jin-seok’s actions escalate under pressure, leaving him to confront the consequences of a crime rooted in desperate circumstances.
Back in the present, Choi, who has survived the crash, intends to poison Jin-seok, but Jin-seok declares that his memories have returned. Choi then reveals that he is the doctor’s son—someone Jin-seok spared two decades ago—whose own family was left behind after a series of tragedies. The revelation shreds Jin-seok’s resolve, and in a moment of moral collapse, he lies and accepts responsibility for the murders, though Choi remains distrustful. Overwhelmed by guilt and the truth he has uncovered, Jin-seok poisons himself and dies. At the same time, Choi dies by suicide, leaping from a hospital window.
In the final flashback to 1997, Jin-seok stands at the river, when a little boy offering a lollipop approaches. Declining, he watches as the boy re-joins his family—who are depicted as the doctor and his kin—and they walk away together, leaving Jin-seok with a quiet, unsettling smile of peace.
