Insomniac detective Jang Hae-jun works in
Busan and only sees his wife, Jung-an, a nuclear power plant manager residing in
Ipo, on weekends. Hae-jun and his partner, Soo-wan, investigate a case involving a retired immigration officer, Ki Do-soo, found dead at the foot of a mountain he often climbed. They interview his much younger wife Song Seo-rae, an emigrant from China who works as a caretaker for seniors. They suspect her because of her lack of displays of grief, a scratch on her hand, bruises on her legs and torso, broken ribs, and a tattoo of Ki's initials resembling the monogram he used to mark his belongings. Hae-jun conducts further interviews with Seo-rae and conducts nightly stakeouts outside of her apartment building, becoming infatuated with her in the process. Seo-rae observes him outside her building and on one of his other investigations. Seo-rae's Monday client says Seo-rae was with her on the day Ki died, and CCTV footage shows Seo-rae entering and leaving the client's home the day Ki died. Questioned about her background, Seo-rae admits that in China she killed her terminally ill mother with
fentanyl pills at her request. Before dying, she told Seo-rae to go to Korea to climb the mountain her Korean grandfather, an independence fighter in
Manchuria, had left her. Seo-rae gives Hae-jun letters written by Ki admitting to corrupt business dealings, including a letter sent to a subordinate that Hae-jun interprets as a suicide note. Hae-jun rules the death a suicide, despite Soo-wan's doubts, and informs Seo-rae that she is no longer a suspect. Seo-rae and Hae-jun go on a date at a Buddhist temple, visit each other's homes, and become close. At his apartment, Seo-rae burns Hae-jun's evidence photos from her husband's case, reasoning that Hae-jun's insomnia is caused by his disturbing cases haunting him. When Hae-jun fills in for a visit to Seo-rae's Monday client, Hae-jun learns that Seo-rae and the client have the same model of cell phone, and that the client has dementia and does not know what day of the week it is. On the client's phone, he sees that the housebound woman apparently walked the distance equivalent to 138 flights of stairs on the day of Ki's death. Hae-jun realizes Seo-rae switched her phone for her client's, snuck out the back window of the house to avoid the CCTV cameras, and climbed the mountain to push Ki off. Confronting Seo-rae in her apartment, he concludes that she also forged the suicide note and, to her consternation, accuses her of getting close to him to destroy his evidence. Hae-jun says Seo-rae has destroyed his pride in his job and that, since meeting her, he has become "shattered"; nevertheless, he has covered up the evidence and instructs her to throw the incriminating phone into the sea before leaving. Thirteen months later, Hae-jun has moved to Ipo to live with Jung-an after developing depression and more severe insomnia. At a fish market with Jung-an, he encounters Seo-rae with her new husband Im Ho-Shin, a business investor. The next day, Ho-Shin is found dead in his mansion's swimming pool. Hae-jun takes on the case and is convinced Seo-rae is the culprit. She admits only to draining the pool so that Hae-jun would not be disturbed by the blood. Sa Cheol-seong, a Chinese immigrant, soon confesses to killing Ho-Shin for defrauding his late mother of her life savings. Sa denies that Seo-rae played any role and reveals he had installed a tracker on Seo-rae's phone so he could find where Ho-Shin lived. Jung-an learns that Ho-shin had called her the day he died. Hae-jun confronts Seo-rae at the mountain her grandfather left her. She rushes at Hae-jun after he scatters her grandfather and mother's ashes at the edge of the mountain, only to hug him as she did at her apartment. She reveals that she kept the phone with the incriminating evidence from Ki's case, and proposes that he use it to "reinvestigate" her. They kiss passionately. Hae-jun returns home to find that Jung-an, who suspects him of having worked with Seo-rae to murder Ho-shin, is leaving him for her coworker. The next day, Hae-jun learns from Sa that Seo-rae visited Sa's mother in the hospital on the day that she died. He concludes that Seo-rae slipped Sa's mother the remaining fentanyl pills she had in her possession, knowing Sa would kill Ho-Shin as soon as his mother died. Hae-jun tracks Seo-rae's phone via the tracker Sa installed and chases her to a beach. Over the phone, she tells him that Ho-Shin discovered a phone recording of Hae-jun telling Seo-rae that he loved her, and that Ho-Shin had planned to expose their illicit relationship. Hae-jun does not recall telling Seo-rae that he loved her, though she says she began loving him the moment he stopped loving her. At the beach, he finds her empty car and the cell phone, which contains the recording of his instructions to Seo-rae to destroy the phone with evidence from Ki's case. Further out on the shore, Seo-rae digs a pit in the sand and climbs into it as the tide rises, letting it drown and bury her. Hae-jun arrives at the beach and is unable to find Seo-rae, unaware that she is buried in the sand beneath him. He searches desperately for her and cries in anguish. ==Cast==