The narrative unfolds with the tragic birth of Lee Geum-hwa and her twin sister, whose existence leads to unexpected consequences. While Geum-hwa enters the world with a deformed leg—her sister's fetus having nourished itself parasitically—she is born into a bleak reality. Their mother tragically succumbs shortly after childbirth, and their father, overwhelmed by despair, takes his own life. As a result, Geum-hwa is raised in a remote village by her grandparents, who relocated there to raise dogs for a living. Meanwhile, her sister, deemed a "thing" by society, is concealed from public view, imprisoned in a shed in the backyard, her existence unregistered and silenced.
In the village, strange occurrences arise, prompting a group of locals, guided by a shaman, to investigate the unusual behavior of the livestock. Their search leads them to the shed, where chaos ensues as snakes escape, inflicting bites on the curious onlookers.
Amidst this, Pastor Park, a determined individual dedicated to exposing cults, embarks on an investigation into a clandestine group known as Dongbanggyo or "Deer Mountain." Meanwhile, law enforcement opens a case regarding the discovery of a girl’s body, horrifyingly interred in concrete. During this time, Gwangmok, an individual tormented by guilt, confesses to his friend Kim Cheol-Jin, a truck driver who feels he has failed his duties to a father figure. Gwangmok reveals the haunting visits he receives from dead children at night, urging him towards oblivion.
Autopsy findings on the deceased girl unveil a talisman within her mouth, prompting the pathologist to recollect similar occurrences from the past. The police trace the girl back to Kim Cheol-Jin through his construction company, but when confronted, he tragically leapt from a rooftop. During this tumultuous period, Gwangmok is handed information about Geum-hwa by a member of Dongbanggyo, fueling his obsession to watch her closely. As nightmares of dead girls plague him, they are often accompanied by a woman singing a haunting melody.
Pastor Park delves into the origins of Dongbanggyo, discovering the scriptures penned by Kim Pungsa. His investigation leads him to the esteemed Nechung Tenpa, a Buddhist master who once sought to meet the cult's founder, Kim Je-seok. Insights reveal that Kim Je-seok made a notable donation to a youth detention center decades prior and had adopted a group of troubled youths—one of whom was the now-deceased Kim Cheol-Jin. Through careful deduction, Park concludes that three of these boys have perished, each depicted as deities in drawings, with the surviving figure identified as Gwangmok.
As the story progresses, Gwangmok visits Geum-hwa's home with sinister intentions, determined to bring about her demise. However, an unexpected interruption occurs when birds flock through the window, diverting his attention. Venturing to the shed, he encounters Geum-hwa's sister, who seizes his leg and drives him away in fear.
Gwangmok subsequently arrives at the Dongbanggyo Residence, speaking to a youthful henchman. There, he learns that Geum-hwa is regarded as "the snake." A confrontation with his fading "Father" reveals memories of being branded as Gwangmok, a protector of the light.
Park engages in a pivotal conversation with Nechung Tenpa, who recounts a significant encounter with the revered Master in 1985. Tenpa claims the Master achieved immortality, showcasing twelve fingers—an identity attributed to Donggon scriptures that foretell a "snake" set to extinguish the light. This impending malevolence is believed to emerge from the Master’s birthplace, with the prophecy linking it to the demise of young girls in the vicinity of Yeongwul during 1999.
In a chilling turn of events, Gwangmok, now having abducted Geum-hwa, contemplates her execution. As Geum-hwa queries the rationale behind her death, he chillingly labels her as evil, yet offers the unsettling notion that her death isn’t final. In a desperate plea, Geum-hwa brings up her sister, asking Gwangmok to kill her as well, dreaming of being reborn as a human in the next life.
Meanwhile, Gwangmok discovers the sister and contemplates the action he intends to take. However, she undergoes a profound transformation, adopting a serene Buddha-like form. In a shocking revelation, she confesses to having awaited his arrival, revealing her role as a comforting presence in his dark dreams. Repeating the same song as in his visions, she implores him to seek a mark from the Father figure he holds dear—six fingers, signifying a crucial prophecy.
Back at the residence, Gwangmok recants his experience to the henchman, only to learn an astonishing truth about his dying "Father," who only possesses five fingers. This revelation culminates in a betrayal as the henchman, revealed to be Kim Je-seok himself, leads Gwangmok into a trap where he is shot. The henchman drives Gwangmok away to eliminate Geum-hwa’s sister.
While all these dark revelations unfold, Pastor Park covertly investigates the residence, confused about the origins of the old man's illness. As he hides, it becomes clear that the ailing figure is but a disciple, with Pastor Park learning the shocking truth behind the cult's manipulation.
Kim Je-seok eventually reveals to Gwangmok the reality of their roles in this grim narrative—he is the light while Gwangmok is but a star meant to guard him. The conversations take a deadly turn, resulting in decisions that entwine fate and morality, leading to a catastrophic car crash that enmeshes both characters in a deadly struggle amidst leaking gasoline.
In a climax filled with desperation, Gwangmok seizes an object gifted by the sister—a lighter—which he ultimately uses to set Kim Je-seok ablaze. In the poignant aftermath, Geum-hwa, still breathing, cradles her sister in her arms as she succumbs to the tragedy of their intertwined fates, marking a bittersweet end to a harrowing tale of reliance, sacrifice, and the search for redemption.
