As the years went by since extinguishing the flames of his troubled youth, Byeong-soo, played by Sol Kyung-gu, had settled into a quiet life as a veterinarian. Here, the most violent encounters involved caring for stray or injured animals. Yet, within him, the shadows of his past simmered, creating a chilling belief that certain people were fated for extinction. His dark obsession led him to a secretive string of killings, where he would bury his victims deep within a hidden bamboo grove, an unnerving reflection of his dance with death.
However, fate took a cruel turn when Byeong-soo suffered a devastating car accident, resulting in a degenerative version of Alzheimer's disease. This progressive illness blurred the lines of reality for him, forcing him to grapple with a disorienting world where even the simplest details, such as his address, eluded him. After one particularly bewildering episode, he wandered into a police station, forging an unexpected friendship with the warmhearted officer Byeong-man (Oh Dal-su), whose presence offered him some comfort.
Byeong-soo's daughter, Eun-hee, portrayed by Kim Seol-hyun, emerged as his greatest source of solace. Motivated by love and concern, she encouraged him to use a recorder to document his daily experiences. In the quiet moments of reflection, he began to pen his jumbled thoughts, creating a memoir that highlighted the ephemeral nature of his fading memories. As he attended poetry classes, he found himself oddly captivated by Jo Yeon-joo (Hwang Seok-jeong), a student whose unsettling interest in his poetic takes on murder intrigued him.
Amidst a backdrop of growing fear following the horrific slayings of two young women in town, Byeong-soo's tenuous grasp on reality began to erode further. When he uncovered a bloody bag in a vehicle, coupled with the driver's suspicious demeanor, he was thrust into a web of paranoia and distrust.
In a frantic attempt to regain a semblance of control over his life, Byeong-soo anonymously alerted law enforcement about his unsettling encounter, unknowingly setting in motion events tied to the true identity of the murderer. As he endeavored to reconcile his fragmented memories with the stark truths of the world around him, he found himself ensnared in a psychopathic killer's deadly game.
This convergence of darkness was symbolized by Min Tae-joo, played by Kim Nam-gil: the very murderer behind the horrifying crimes. As Byeong-soo's sanity faltered, he became torn between the yearning for retribution and the overwhelming burden of his own mortality.
In a chilling turn, Byeong-soo awakens bound in his own bedroom, with the menacing presence of Tae-joo looming over him like a phantom of despair. The killer chillingly discloses the tampering with crucial evidence and issues a dire ultimatum—Byeong-soo must take the blame for the atrocities or face dire consequences for Eun-hee. In a desperate bid for survival, Byeong-soo lunges for a bag as a makeshift weapon, but Tae-joo's deceptive cunning and brute force leave him reeling and helpless on the ground.
Tranquilized and incapacitated, Byeong-soo regains consciousness with little memory of his harrowing experience. His immediate concern is for Eun-hee’s safety, prompting him to send her to the convent where his sister Maria (Gil Hae-yeon) lives for her protection.
Undeterred, Byeong-soo relentlessly tracks Tae-joo to a deserted shack, where he uncovers a disturbing collection of footage revealing Yeon-joo's perilous condition. Armed with this evidence, he approaches the authorities, only to face skepticism and the growing insinuation that he himself might be the perpetrator of the gruesome acts.
As pressure mounts, Byeong-soo grapples with the fear of losing Eun-hee forever. His attempts to reach Maria are thwarted when painful memories surface about her tragic fate, complicating his already fragile state of mind. The police's excavation of the bamboo grove yields a horrific discovery—the remains of multiple victims, including Yeon-joo.
Back home, the remnants of Byeong-soo's shattered mind begin to surface as past traumas are unearthed. He recalls the haunting words of his estranged wife, suggesting that Eun-hee might not even be his biological child, an unsettling revelation that plunges him deeper into uncertainty regarding his family's history.
Following a devastating accident, the psychological toll culminates in a debilitating confusion, leading him to believe he may have unintentionally taken Eun-hee’s life—a burden too grotesque to bear, spiraling him towards thoughts of self-destruction.
Yet, fortune intervenes unexpectedly when Byeong-soo accidentally captures Tae-joo’s confession during their initial confrontation, reigniting his will to fight. Driven by newfound determination, he reaches out to Byeong-man, formulating a strategy to unveil the true villain.
As the narrative unfolds, Byeong-soo and Byeong-man converge on an isolated cabin, unaware they are walking into a trap laid by Tae-joo. A tense showdown ignites, escalating to a visceral struggle that tests the limits of their endurance. Just when all seems lost, Eun-hee's timely arrival proves pivotal, assisting Byeong-soo in overpowering and ultimately ending Tae-joo's reign of terror.
In the wake of this brutal clash, Eun-hee grapples with a painful truth: her mother’s remains have been discovered in the bamboo grove. As Byeong-soo strives to reassure her that she is not defined by Tae-joo's wickedness, he himself finds an intricate web of justice awaiting him, alongside repercussions that threaten to engulf them both.
As Eun-hee closely examines a haunting final recording left by Byeong-soo, she is washed over by a cleansing wave of emotion, leading her to confront her grievances and offer forgiveness. However, a reunion with the fragile Byeong-soo elicits unexpected turmoil; he mistakes her for Maria, his memories drifting amidst the ravages of time.
In a fleeting moment of clarity, Byeong-soo wrestles with the desire to end his own life, battling the instinctual urge that compels him to confront the legacy of his past—a legacy filled with harrowing darkness.
Caught in a poignant scene of inner turmoil, Byeong-soo stands at the edge of a train tunnel, gripping a locket that bears Tae-joo's likeness, confronting the specter of his daughter's former love. As an eerie smile twists the shadows around him, a haunting realization takes root: "Never trust your memories, for Min Tae-joo remains very much alive."
