Oh Soo-Min is a dutiful young Catholic priest, filled with energy. He is a member 643 Regia. The group performs exorcisms unofficially. In the past, his own mother was possessed by a devil. She could not receive an exorcism and she consequently died. After that, Oh Soo-Min was taught by Priest Moon Ki-Sun on how to perform exorcisms. Moon Ki-Sun is the person who formed 643 Regia.

Meanwhile, Ham Eun-Ho became a doctor after her family went through an unfortunate accident. She wants to save the lives of people. Now, Ham Eun-Ho works in the emergency room of a hospital. Ham Eun-Ho does not believe in God, but she witnesses a supernatural phenomenon and meets Priest Oh Soo-Min. She faces a big change in her life.