Se-jin Oh, a solitary young professional, resides in a high-rise apartment building situated in a neighborhood of Seoul. To pass the time, she often finds amusement in watching her neighbors through binoculars. One fateful night during the Christmas season, while riding the subway home, she witnesses a woman clad in red attempt to throw herself in front of the train, trying to drag Se-jin down with her. Though the incident leaves a mark on Se-jin, she is unknowingly haunted by the spirit of the deceased woman. A peculiar pattern emerges as the lights across the way flicker each night at exactly 9:56 PM, often heralding yet another tragic suicide.
Se-jin forges a bond with Yu-yeon, a woman confined to a wheelchair and victimized by her caregivers—specifically those who are among the unfortunate souls who have committed suicide. Yu-yeon presents Se-jin with a puzzle cube, suggesting that it serves as a distraction from the emotional turmoil they both experience. Se-jin fervently attempts to persuade her neighbors to keep their lights on until the clock strikes ten, resulting in tension with Detective Yang Na-sun, who begins to investigate the strange sequence of events. He discovers a troubling connection: many of the victims possess identical keys to a mysterious apartment.
The apartment in question, 704, belongs to a reclusive individual named Shin Jung-soo, who initially resists scrutiny. When confronted by Detective Yang, Shin denies any involvement in the string of deaths, displaying aggressive behavior instead. Meanwhile, Se-jin also befriends Jung-hong, a student neglected by her parents during their absence. Se-jin hands the puzzle cube over to Jung-hong, who realizes that the numbers inscribed on the cube align with the apartment numbers of known victims—except for one.
Delving deeper, they stumble upon an article revealing that Yu-yeon had taken her own life a year prior. Driven by curiosity, Se-jin visits apartment 1203 for further investigation, only to find another corpse, this one with a key to 704 in its mouth. In this chilling setting, Se-jin experiences the grim realities of Yu-yeon's life; orphaned after a car crash killed her parents, Yu-yeon was initially cared for by her neighbors, who later succumbed to resentment and mistreatment, subjecting her to horrific abuse that ultimately led her to take her own life, her blood staining her dress a deep red.
As the mystery unfolds, the desperate Shin Jung-soo, horrified at the revelations, takes his own life as well. Jung-hong, having seen her parents return, races to prevent them from being caught in the night’s fate at 9:56 PM. Simultaneously, Yu-yeon confronts Se-jin, filled with vengeance for turning her back on her like so many others. In a tragic turn of events, Se-jin flees to the rooftop, pursued by the furious spirit. To end the cycle of hate and suffering, Se-jin permits Yu-yeon to possess her, leading her to leap from the building—a horrifying scene witnessed by both Detective Yang and Jung-hong.
Two months later, as Jung-hong assists Detective Yang in moving into an apartment within the same building, they are again confronted by the spectral presence of Se-jin. As the clock strikes 9:56 PM, the lights flicker ominously, leaving them in a haunting remembrance of the events that had transpired.
