An insomniac detective named [Hae-Jun](teo-yoo) is stationed in the thriving city of Busan, where he sees his wife, [Jung-An](junghyun-lee), a diligent nuclear power plant employee, only once a week due to his grueling work schedule. Together with his partner [Soo-Wan](hakjoo-lee), Hae-Jun delves into the peculiar case of a retired immigration officer, [Ki Do Soo](seungmok-yoo), whose lifeless body is discovered at the base of a mountain he frequently ascended. As they investigate, they question the victim's significantly younger wife, [Seo-Rae](wei-tang), who immigrated from China and works as a caregiver for the elderly. Suspicion falls upon her due to her oddly muted reaction to the tragedy, the visible injuries on her body, and a tattoo bearing Ki's initials, reminiscent of how he marked his possessions.
Driven by an unsettling fixation, Hae-Jun conducts nightly surveillance outside Seo-Rae’s residence, consumed by thoughts of her and imagining life within her apartment. Seo-Rae, not oblivious to his presence, begins to follow his other cases. A client of Seo-Rae's provides an alibi, claiming she was with her during Ki's fatal incident. In a moment of dark revelation, Seo-Rae interviews her troubled past, confessing to having hastened her mother's death with fentanyl, fulfilling her terminal wishes. Her mother instructed her to venture to Korea to claim the mountain left to her by her grandfather, a heroic figure from the independence movement in Manchuria.
Hae-Jun and Soo-Wan unearth incriminating letters from Ki, revealing his corruption as an immigration official and hinting at his impending suicide. As Hae-Jun concludes that Ki’s death was self-inflicted, he informs Seo-Rae that she is no longer under suspicion. The two then embark on a heartfelt connection, visiting a Buddhist temple and sharing moments in each other's homes. Seo-Rae meticulously destroys evidence from her husband's case, believing it to be a source of Hae-Jun's insomnia, burdened by unresolved cases.
However, when Hae-Jun substitutes for her client, an unsettling discovery unfolds; both Seo-Rae and her client possess identical smartphones, raising red flags in Hae-Jun's mind. He learns the elderly woman had purportedly conquered 138 flights of stairs on the day of Ki's demise, leading him to surmise that Seo-Rae swapped their phones on that fateful day. Soon, the depth of Seo-Rae's deception becomes painfully clear—she had plotted Ki's murder and crafted a phony suicide note to shield her actions. Hae-Jun tells her she has shattered his professional integrity and feels "broken" since their paths intertwined. He implores her to discard the damning phone into the ocean.
Thirteen months later, Hae-Jun relocates to Ipo, where he lives with [Jung-An](junghyun-lee), wrestling with depression and crippling insomnia. While visiting a fish market, he unexpectedly encounters Seo-Rae, now married to [Im Ho-Shin](yongwoo-park), a business tycoon. Soon after, Ho-Shin's body is found in the mansion's swimming pool, and Hae-Jun can't help but suspect Seo-Rae's involvement. She nonchalantly claims that she drained the pool so Hae-Jun wouldn’t be disturbed by the mess. A Chinese immigrant, [Sa Cheol-Seong](seo-hyunwoo), confesses to killing Ho-Shin, allegedly avenging his mother's financial ruin at Ho-Shin's hands. Despite Sa's claims of Seo-Rae's innocence, Hae-Jun discovers she had visited Sa's dying mother, leading him to believe she injected the final dose of fentanyl into her.
The strain of suspicion takes a toll on Hae-Jun's marriage, prompting [Jung-An](junghyun-lee) to leave, suspecting an affair with Seo-Rae. Hae-Jun confronts her at the mountain left to her by her grandfather, where she unveils a shocking truth—Ho-Shin had threatened to reveal their hidden relationship after discovering a recording of Hae-Jun professing love to Seo-Rae. Although Hae-Jun fails to recall his words, their emotions escalate, culminating in a passionate kiss at the mountain's peak.
As Seo-Rae drives to the sea, Hae-Jun uncovers the phone containing the questionable recording, in which he urged her to destroy evidence from Ki's case. At the beach, Seo-Rae digs a trench in the sand and willingly submits herself to the rising tide, choosing to become an eternal enigma for Hae-Jun. When he finally arrives, he is unable to perceive her presence, standing unknowingly on her unmarked grave. In a desperate search, he calls out for her, overwhelmed by despair.
