The son of a North Korean spy decides to follow in his father’s footsteps to protect his little sister. After his father’s botched espionage mission, North Korean Myung-hoon and his young sister Hye-in 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, and rescues her when she comes under attack. South Korean Intelligence soon discover Myung-hoon’s activities and begin tracking him, all the while his own government sends a vicious assassin to eliminate him.

Na-mi, a transfer student, becomes a laughingstock on her first day of school because of her strong southern dialect. Six girls jump in to help the new kid adapt to the new environment, and the girls come to call their circle of friends “Sunny.”