After his father's botched espionage mission, North Korean Myung-hoon (Choi Seung-hyun, aka T.O.P) and his young sister Hye-in (HAN Ye-ri) are sent to a labor prison camp. In order to save his sister's life. Myung-hoon volunteers to become a spy and infiltrates the South as a teenage defector. While attending high school in the South, he meets another girl named Hye-in (KIM Yoo-jeong) and rescues her when she is attacked. South Korean Intelligence soon discovers the plot and begins closing in on Myung-hoon, while his own government sends a vicious assassin to eliminate him.