Nineteen-year-old Li Myung-hoon (T.O.P (entertainer)) never imagined he would become a killer. Born to a privileged life in North Korea, his dream was to become a pianist. But when his father, a North Korean spy, dies disgraced, Myung-hoon and his younger sister Hye-in (Kim Yoo-jung) are sent to a "guilt-by-association" Prisons in North Korea. Their father's superior, high-ranking military official Colonel Moon (Jo Sung-ha) proposes a deal to Myung-hoon: if he goes down to the South as a "technician" (an assassin) and finishes what his father had failed to accomplish, he and his sister will be released from the prison camp. Myung-hoon accepts the deal and undergoes two years of intense training.