Looking for a fresh start, the young taxi driver Yong-hyun [Kim Myung-min] moves into Apt. 504 of Migum Apartments, an aging tenement whose walls seem to whisper of the past. The moment he settles in, he meets his neighbor, Mr. Lee [Gi Ju-bong], a divorced writer, whose presence hints at unfinished stories echoing through the building. On the first night, Hyun begins his night shift after a quick stop at a nearby store where another neighbor, Sun-yeong [Jang Jin-young], works, introducing a small circle of residents whose lives will intertwine in surprising ways.
The next day, an old friend visits, and Hyun asks about Mee-jung, a woman who borrowed money in his name and fled, a reminder of the fragility and shadows that hover over his uncertain start. That night, he spots Yeong [Kim Joo-ryoung] on the street and offers her a ride home. Inside her apartment, an abusive estranged husband barges in, assaults her, and leaves with her money, leaving Hyun to decide what help to offer. He checks on Yeong and finds her on the rooftop, seemingly about to jump, but his presence keeps her from taking that plunge, a moment that binds their fates more tightly than either expects.
Yeong’s friend Eun-soo [Jo An], a music teacher, visits her at work and mentions recurring dreams of her late boyfriend, hinting at the sorrow and guilt that haunt Yeong. The following day, Yeong and Soo run into Hyun at the theater, and together they wander through nearby spots, where Yeong reveals that Kwang-tae (the previous tenant of Hyun’s apartment and Soo’s boyfriend) died there, a fact that threads the past into the present. Returning from his shift, Hyun finds a hysterical Yeong—she has just killed her husband, who came back for more. Hyun buries his body in the mountains and tries to calm Yeong, a groomed harbor of fragile trust that soon deepens into a dangerous closeness.
Yeong reveals she never divorced her husband, driven by a fear of loneliness after her mother’s death and her father’s decline. As Hyun gets a haircut at the barbershop, Lee and the barber comment on a photo of a couple on the wall, revealing that they lived in Hyun’s building 30 years ago. The story Lee shares shocks Hyun: a husband who had an affair with the woman next door murdered his wife, fled with the other woman, and abandoned his infant son; a stove fire later exposed the hidden tragedy. Hyun is stunned by a birthmark on his own body that resembles the past, making him suspect a connection to the misfortunes in the building. Lee also reveals that Yeong has lost her child and that Kwang-tae came to the building for inspiration to write a novel. He recalls Tae’s death in a mysterious fire, noting flames that consumed only Tae’ s body while the rest of the room stayed intact, a detail that fuels the building’s eerie rumor of a curse rooted in the murdered mother.
Police question Hyun about Mee-jung’s disappearance, adding pressure to the fragile balance in his life. While helping at Hyun’s place, Yeong discovers a hidden bag of jewelry in the fridge and secretly takes a ring, a theft that Hyun remains unaware of at first. Later, she gifts Hyun a muffler, and soon after, he notices the ring is missing and realizes Yeong stole it, a revelation that strains trust between them.
Lee burns Tae’s notes but is confronted by Soo, who accuses him of stealing them from Tae’s apartment instead of aiding him during the fire. Soo suspects Lee was involved in Tae’s death, but Yeong disagrees. Soo warns Yeong that Hyun may be dangerous and accuses her of using him; Yeong admits she doesn’t care about Hyun’s background and needs comfort, not love. Overhearing this, Hyun becomes furious.
Soo later recalls being pregnant with Tae’s child and how he once mentioned a woman’s ghost in his apartment who said her child was coming, a memory that adds to the building’s haunting atmosphere. After days of avoiding each other, Hyun invites Yeong on a trip. Both are intoxicated, and Hyun reveals how a school bully relentlessly mocked his orphanhood, a confession that ties his violence to a lifetime of wounds.
At a motel, Yeong is haunted by flashbacks of her son suffocating to death in a cupboard while hiding from her husband’s assault, and she later buried him in secret. She decides to leave, but Hyun confronts her for using him like Mee-jung and demands the stolen ring. The argument escalates when Hyun taunts her for being an orphan, and in a fit of fury, he strangles her to death with the muffler she had gifted him.
As he prepares to bury Yeong, he finds an old photograph of her with her parents in her purse. To his horror, the man in the photo resembles the man in the couple’s photo at the barbershop. To be sure, he breaks into the barbershop and realizes they are indeed the same.
Paranoid and shattered, Hyun returns to the building and finds a disheartened Lee, devastated by the rejection of his novel by publishers. Knowing that Lee depicted him as the ghost’s son in the novel, Hyun, consumed by rage, violently beats Lee nearly to death. Determined to leave, Hyun gathers his important belongings.
As Hyun exits the building, he hears faint cries of a baby, causing him to stop. Turning around, he experiences something inexplicable as a mother’s lullaby begins to echo. Overcome with emotion, he breaks down. In the final shot, from a distance, Hyun is seen facing a faint image resembling a woman.
