Deacon Choi and Father Kim visit Youngshin who is in a vegetative state. Their plan is to exorcise the demon that has settled into her body. The demon, however, is a formidable force and Youngshin shows no sign of improvement despite Father Kim and Deacon Choi's efforts. The demon actually grows stronger by attacking Deacon Choi's weak mind for this is his first time here.
Choi is 'the 12th assistant deacon' to Father Kim, proof that the demon that has taken over Youngshin's body is a formidable one. The demon feeds off the fear that human beings harbor and this connection demonstrates the vices of Korean society.
Youngshin seems to have attempted suicide because of the pressure of the college entrance exam while Deacon Choi still suffers from the agony of having been beaten while serving in the military. Korean society, which does not take care of its individuals, provides a favorable environment for demons to run rampant. Therefore, the activities of those who strive to fight against them are essentially lonely. This is what makes 12th Assistant Deacon fundamentally different from The Exorcist, which it seems to resemble on the surface. (Huh Namwoong)

After tracing the origin of a disturbing supernatural affliction to a wealthy family's ancestral gravesite, a team of paranormal experts relocates the remains—and soon discovers what happens to those who dare to mess with the wrong grave.

To save a girl in danger, a priest and deacon jump into a mysterious case.