Lee Myung-hoon (Choi Seung-hyun)'s father was a South Korean spy who died in injustice because of being framed. As his children, Lee Myung-hoon and his sister Hye-in (Kim Yoo-jung) were treated roughly by the authorities. The two suspected of collaborating with the enemy were imprisoned and in a very bad situation. In order to protect his beloved sister, the kind and upright Lee Myung-hoon was forced to take over his father's job and became a ruthless spy. He received cruel and arduous training, and then disguised himself as a high school student to sneak into South Korea, just to assassinate a technician named Bukdusung. At school, Lee Myung-hoon met a girl named Hye-in (Han Ye-ri), and what Lee Myung-hoon didn't expect was that the two gradually fell in love. However, Lee Myung-hoon knew that his identity was special, and the relationship between the two would not have a good result. Not only that, his existence would even bring disaster to Hye-in.