This is a pre-produced drama. Filming was completed on 2017-March-12.
When a slacker city kid Lee Shi Kyung (Kai) and his sister, Lee Shi Young (Lee Ye Hyun) are suddenly transferred to a high school in the countryside, they have trouble adjusting to their strange new surroundings. But not without the help of a new friends, Park Ga Ram (Baek Chul Min), the town’s golden boy, and a mysterious girl, Kim Bom (Kim Jin Kyung).
Seungri High School ranks as one of the worst of the 178 high schools in Seoul based on academic scores. Seungri High School is now busy preparing presentations for its new students. Class 2 is at the bottom of grade 2 at Seungri High. Nam-Soon (Lee Jong-Suk) is elected class president for grade 2, thanks to the support of Jung-Ho (Kwak Jung-Wook), who is a member of the school gang.
Se-Chan (Daniel Choi) is the top Korean language teacher at a famous institute in Gangnam. In order to improve the student's scores at Seungri High School, the school hires Se-Chan. When Se-Chan appears at the school's opening presentation, Nam-Soon becomes embarrassed.
Nothing is more symbolic of the division of Korea into two countries as the 'Bridge of No Return' in Panmunjom, the demilitarized zone on the border between the two states. One day, a border guard is killed by a rifle bullet; the suspected marksman is a South Korean soldier, who is found wounded in the middle of no-man's-land. This incident will have grave repercussions. Both North and South Korea regard the incident as an act of deliberate provocation. The North accuses the South of having committed a 'terrorist attack', whilst South Korea suspects the North of having attempted an 'abduction'. Both sides appeal to the authorities of the neutral states (NNSC) for help and ask them to investigate the affair.
A female Swiss army captain of Korean descent is then sent to Panmunjom to investigate the death of the North Korean border guard, apparently shot in the line of duty by a South Korean guard. At first, neither army officers from the South nor communist officials from the North will cooperate - until it is obvious to all that the truth must come out to defuse a potentially explosive international incident.
A series of black garbage bags filled with dismembered body parts begin to show up around the city: in an elevator, on a highway, at the edge of a basketball court, and rendering the victims nearly impossible to identify. The killer's motive is a complete mystery. Leading the investigation is Detective Cho, a burnt-out cop trying to redeem himself with this case. Setting up a special squad dedicated to these gruesome murders, Cho and his partner, the veteran Detective Oh, map out the order of the murders, interchanging the parts like pieces in a grotesque puzzle. Mystified by the surgical precision of the dismemberments and finding a medical antiseptic called "Hexamedrin" spread all over the bodies, the detectives start suspecting a doctor or a biologist as the killer.
A break opens up when Cho is led to a beautiful and enigmatic young artist, Chae Su Yeon, whom all the victims have had relations with. But besides this revealing breakthrough, Cho is far from solving the case. Still not having found a motive, Cho is grasping for many clues. As the murders continue, Cho is drawn closer to Su Yeon and begins unravelling her tortured past, leading to new questions and suspects. Cho and Oh subsequently discover that her father, a famous painter, mysteriously disappeared years before. Cho decides to investigate her late college classmate and current colleague Ki Yeon, and her friend, Sung Min.
While Cho questions Ki Yeon, the fragile Su Yeon is attacked but manages to escape. After being released from police custody, Ki Yeon disappears, and the next morning Detective Cho is notified of the fourth murder. Under the rainy urban landscape of Korea's capital, another garbage bag is discovered on a highway. This time, the corpse is a head-less Ki Yeon.
