Born in a family of discipline, Luo Yaohua (played by Chen Hao) has been determined to join the police since he was a child, but his mother does not want him to repeat the mistakes of his deceased father. She hopes that he can become an ordinary middle school teacher. As his mother wished, Yaohua became a teacher at Lam Tin Kwong Memorial School, a third-rate middle school in Kowloon City. The four domineering campus villains K4 set up a trap to frame Yaohua. Yaohua resigned angrily and decided to fulfill his dream of becoming a policeman. However, fate makes people play tricks on people. Yaohua, who had just joined the police force, was sent back to his original school as an undercover to investigate campus gang drug incidents. The new teacher in the school, He Miaoxue (played by Tavia Yeung), hopes to guide the students back on track. Her persistence and sincerity deeply touched Yaohua. However, in order to solve the case, Yaohua had to pretend to pursue the cold-looking bowling coach Gu Jialan (played by Linda Chung), and the young and promising inspector Cheng Wenli (played by Ron Ng) also pursued Miaoxue. The campus fashion TV series "Why Sir is Sir" launched by Hong Kong Television Broadcasting Co., Ltd. is starred by first-line stars such as Moses Chan, Tavia Yeung, Ron Ng, Linda Chung, etc. This drama is one of the TVB program tour series in 2012.