An insomniac detective, Hae-Jun, works in Busan and only sees his wife, Jung-An, a nuclear power plant worker residing in Ipo, once a week. Hae-Jun and his partner Soo-Wan encounter a case where a retired immigration worker, Ki Do Soo, is found dead at the foot of a mountain he often climbed. They interview his much younger wife, Seo-Rae, an emigrant from China who works as a caretaker for seniors. They suspect her because of her insufficient displays of grief, a scratch on her hand, bruises on her legs and torso, and a tattoo of Ki's initials in the manner that he marked his other belongings.Hae-Jun does nightly stakeouts outside of Seo-Rae's apartment building, becoming obsessed with her and imagining himself inside her apartment. Seo-Rae discovers him stalking her and tails his other investigations in turn. Seo-Rae's Monday client says that Seo-Rae was with her on the day that Ki died. Seo-Rae admits that in China, she killed her mother with fentanyl pills because her mother was terminally ill and asked her to do so. Before dying, she told Seo-Rae to go to Korea to claim the mountain her Korean grandfather, an independence fighter in Manchuria, had left her. Soo-Wan and Hae-Jun discover letters from Ki admitting to being a corrupt immigration official and a letter sent to Ki's subordinate that they interpret as a suicide letter. Hae-Jun concludes that the death is a suicide 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. Seo-Rae burns the photo evidence from her husband's case, reasoning that Hae-Jun's insomnia is caused by his haunting cases. One day, she has Hae-Jun substitute for her at her Monday client. Hae-Jun learns that Seo-Rae and the client have the same model of cell phone, and the client has dementia and does not know the day of the week. On the client's cell phone, he sees that the housebound woman apparently walked up 138 flights of stairs on the day of Ki's death. Hae-Jun realizes that Seo-Rae switched phones with her client on the day of the murder and then climbed the mountain to push Ki off. She also forged his suicide note and got close to Hae-Jun to destroy his evidence. Hae-Jun tells Seo-Rae that she has destroyed his pride in his job and that, since meeting her, he has become "shattered", but instructs her to throw the incriminating phone into the sea.Thirteen months later, Hae-Jun has moved to Ipo to live with Jung-An after developing depression and more severe insomnia. At the fish market with Jung-An, he encounters Seo-Rae with Im Ho-Shin, her new husband, a business investor. The next day, Ho-Shin is found dead in his mansion's swimming pool. Hae-Jun 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 millions of dollars. Sa denies that Seo-Rae played any role and reveals he had installed a tracker on Seo-Rae's phone so he knew where she was at all times. Hae-Jun learns that Seo-Rae visited Sa's mother in the hospital and the mother died later that day. He concludes that Seo-Rae slipped the mother the remaining fentanyl pills she had in her possession, because Sa had threatened to kill Ho-Shin as soon as his mother died.Jung-An leaves Hae-Jun, suspecting an affair with Seo-Rae. Hae-Jun confronts Seo-Rae at the mountain her grandfather left her. She tells him that Ho-Shin had discovered a phone recording where Hae-Jun told Seo-Rae that he loved her and he had threatened to expose their illicit relationship. Hae-Jun doesn't recall telling Seo-Rae that he loved her, though she says she began loving him as he stopped loving her. She reaches Hae-Jun at the edge of the mountain and hugs him. They kiss passionately.The next day, while Seo-Rae drives to the beach, Hae-Jun finds the cell phone with the recording. In it, he tells Seo-Rae to destroy the phone with incriminating evidence from Ki's case. At a beach, Seo-Rae digs a pit in the sand and gets in it as the tide rises, letting it drown and bury her, so that she will forever remain an unsolved mystery in Hae-Jun's mind. Hae-Jun arrives at the beach and is unable to find Seo-Rae, although he is actually standing on her grave. He searches desperately for her and cries in anguish.