YOO SI JIN (played by Song Joong Ki): 33 years old, the chief spy of Taibai Army. A man who studies well and is good at sports. Although he can do many things, he chose the Army NCO School. There is a belief in protecting children, the elderly and beauty. High school students are afraid of smoking on the road, but they will also have the courage to say a few words. Even if the gun is pointed at the temple, you can't do what you shouldn't do. One day, he met Mu-Yeon Kang, a critical and lively doctor.