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There is a supernatural element that is not well explained. Other than that it is as good as any other Lee Dong Wook drama.
I hope my summary/review is not too spoiler-ish. The story follows the lives of the police officers who go undercover. Nobody gets a happy ending and their jobs are their lives. The lines between good and evil get crossed as corruption feeds itself from the inside of the very police force which is supposed to fight it. Characters are believable and human and we keep rooting for them to turn their lives for the better.