讲述了两名天主教司祭被卷入事件中以及由此展开的一系列故事。
金司祭（金允石 饰）因总是擅自行动而被教会其他司祭所排挤，更为了拯救因一宗神秘交通事故而陷入危机的少女英申（朴洙垣 饰），在所有人的反对和怀疑中，做出了危险的选择，独自计划着所有的行动。神学院学生出身的崔副祭（姜栋元 饰），虽然曾是作弊又喝酒的不良学生，但具备了可以帮助金司祭的所有条件，于是他也加入到金司祭的计划中，帮助他一起拯救少女。他并不相信金司祭所作的一切，在帮助他的同时也担负起监视他的任务，与金司祭一起进行危险的仪式，却被卷入了意料之外的神秘事件。

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)