In the vibrant city of Seoul, a troubling wave of supernatural events unfolds, leading to ritualistic murders, unexplained possessions, and widespread panic that grips the population. The chaos stems from a clandestine organization known as the Black Mass, a cult dedicated to resurrecting an ancient demon called Asmodeus. Their sinister rituals disrupt the balance of the city, leaving the local law enforcement utterly powerless against such menacing forces. In search of help, they reach out to a formidable team known as Holy Night, a specialized demon-hunting unit.
The Holy Night comprises three distinctive members. At the helm is Ba Woo, a fierce fighter endowed with incredible strength. Tormented by a tragic past mission, he is propelled by an unwavering commitment to justice. Accompanying him is Sharon, a calm yet formidable exorcist who serves as the team’s spiritual anchor. With her profound ability to sense and drive out malevolent spirits, she channels her powers through ancient rituals and her deep-seated faith. The third member, Kim Gun, is a young tech-savvy genius whose analytical mind and research capabilities provide invaluable support to the team, even though he does not possess any supernatural gifts.
When a young girl named Eun-seo starts showing alarming signs of possession, her elder sister Jung-won, a skeptical neuropsychiatrist, seeks the help of Holy Night. Traditional medical treatments fail, pushing Jung-won to confront her disbelief in the supernatural as her sister's condition worsens. The investigation reveals that Eun-seo is being targeted by the Black Mass cult to become the vessel for Asmodeus during a perilous summoning ritual.
As Holy Night delves deeper into the mystery, they unravel a terrifying sequence of disappearances, violated churches, and hidden ritual sites scattered across Seoul. They encounter fierce confrontations with cult adherents, navigate visions aimed at destabilizing their sanity, and endure emotional and physical attacks meant to undermine their unity. Ba Woo battles his haunting past, while Sharon grapples with her diminishing faith, and Kim Gun discovers his estranged father’s involvement in the cult's earlier activities.
As the cult gears up for the climactic ritual beneath a dilapidated cathedral, time is of the essence. Holy Night launches a desperate incursion to thwart the summoning. A violent confrontation erupts; Ba Woo faces the cult's possessed guardians, while Sharon engages in a spiritual duel with the high priest, and Kim Gun works to dismantle the protective barriers shielding the ritual. Caught fully under the sway of Asmodeus, Eun-seo poses a dire threat to the city.
In a courageous act of self-sacrifice, Ba Woo allows a portion of the demon's essence to possess him, using his body as a temporary host. Guided through a final exorcism by Sharon, he redirects the demon’s might to obliterate the summoning circle. The resulting surge of energy nearly claims their lives, yet it succeeds in banishing Asmodeus back to the underworld, ultimately liberating Eun-seo.
While the cult lies in ruins, the aftermath leaves lingering scars. Eun-seo embarks on a slow path to recovery, her memories fragmented. Jung-won emerges as a believer and decides to join Holy Night in their future endeavors with her medical expertise. The battle-worn trio stands ready, aware that the threat of evil never truly vanishes. In a haunting conclusion, Holy Night receives a new enigmatic file, hinting at the resurgence of dark forces awaiting them.
