The most heartbreaking dynamic is between Dong-sik and Joo-won. Joo-won enters Manyang convinced he is the righteous angel come to punish the devil. But to catch the true killer, he has to learn to think like Dong-sik. He has to break rules. He has to lie. He has to torture a suspect. By the middle of the series, Joo-won looks in the mirror and sees the very monster he came to destroy.

Beyond entertainment, the phrase invites deep academic and philosophical discussion regarding the limits of human morality and the definition of "evil" itself.

What unfolds is a deadly game of cat-and-mouse. Joo-won moves into Dong-sik’s basement. They are forced to work together. And as new bodies begin to fall following the old pattern, the line between hunter and prey dissolves.

Lee Dong-sik (Shin Ha-kyun), a disgraced former detective now working menial tasks at the Manyang police station, has been haunted for two decades by the disappearance of his twin sister, Lee Yoo-yeon. Enter Han Joo-won (Yeo Jin-goo), a young, elite, by-the-book detective from Seoul who is transferred to Manyang to uncover a hidden corruption case. Joo-won immediately suspects Dong-sik is the killer. What follows is not a simple chase, but a psychological wrestling match: the two are forced to work together, each trying to trap the other while a new murder—eerily mimicking the past—rocks the town.