A former doctor turned Catholic priest, Oh Soo-min, performs exorcisms as a member of a secret and unofficial group known as 634 Regia. Twenty years before the events of the series, a demon possessed his mother. Because his father refused to believe in exorcism, she was never saved, and Soo-min witnessed her violent death at a young age. This trauma led him to be taken under the wing of a senior priest, Moon Ki-seon, the founder of 634 Regia, who trained him to become a warrior against demonic forces.

The story is set in and around a Catholic hospital in Seoul, where modern medicine and ancient rites collide. Doctor Ham Eun-ho works in the hospital’s emergency room, a physician who lost her faith in God when her parents died in front of her as a child. She is pragmatic and relies solely on science to save lives. Her world is shattered when a young boy named Woo-joo arrives at her ER with internal bleeding. Despite her best efforts, the surgery fails, but the boy miraculously recovers due to a supernatural force, revealing a demonic possession. When Soo-min arrives at the hospital to perform an exorcism, Eun-ho stops him, but she is soon forced to confront the reality of evil spirits when the demon attacks her colleague.

As Soo-min and Eun-ho clash over faith and medicine, they must cooperate to hunt the entity, which jumps from body to body, leaving a trail of chaos through the hospital. Their investigation involves Detective Koo, who is forced to hide the existence of the exorcist group after a possessed victim dies. The demon is eventually revealed to be an ancient, powerful evil that can manipulate dreams and memories. In a climactic turn, the demon possesses Eun-ho herself. Soo-min is faced with an impossible choice: kill the demon and the woman he has come to care for, or risk letting the evil escape.

The narrative explores the backstories of the main characters, including the tragic events of 1999 that linked Soo-min, Father Moon, and Eun-ho in ways they did not remember. Father Moon, the leader of 634 Regia, faces internal church politics and threats of excommunication as he guides his team through a series of exorcisms, including entering the subconscious of the possessed to pull the demons out from within. The series concludes with the group racing to seal the demon back into a mother-of-pearl box, culminating in a final confrontation where the fate of Eun-ho’s soul hangs in the balance.