Nineteen-year-old Ri Myung-hoon (Choi Seung hyun) 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" forced labor camp. 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.
Myung-hoon finally arrives in South Korea under the guise of a North Korean defector. He is adopted by a South Korean couple who are actually North Korean spies and enrolls at a local high school. He gradually befriends Hye-in (Han Ye-ri), a bullied schoolgirl who shares the same name as his sister and has aspirations of becoming a professional dancer. Myung-hoon then receives his mission: in order to rescue his sister and go back home to the North, he must locate and take out "Big Dipper" (Jung Ho-bin), a North Korean agent working for the opposing government faction. Meanwhile, a power struggle ensues in North Korea with the failing health of dictator Kim Jong-il, and Myung-hoon quickly becomes a liability and must ultimately cope with Colonel Moon's treachery.