Sun Zibing (Ronald Cheng) nicknamed Fa Zai lacks a goal in life. He can only act as a small fry in the gangster gang and do nothing all day with his friend Zhu Peng (Lam Chi Chung). One day, policewoman Zhuang Qing (Fiona Fung), A Sha (Eric Yeung) and 36 (Ella Koon) found Fa Zai. It turned out that Fa Zai's real identity was an undercover police officer. Fa Zai forgot his real identity because of amnesia. Fa Zai didn't believe Zhuang Qing's words at all, but for the meager undercover reward, Fa Zai agreed to help Zhuang Qing. Unexpectedly, Fa Zai and the boss Du Qifeng (Ronald Cheng) hit it off at first sight and became sworn brothers. Zhuang Qing was very disappointed, and Zhuang Qing's tears shocked Fa Zai. With the encouragement of Zhu Peng, the two decided to become a righteous man. Du asked Fa Zai to protect his wife Shuxian (Pace Wu). Fa Zai had a crush on Shuxian for a long time, but Shuxian asked Fa Zai to take her away. Fa Zai faced Shuxian and Zhuang Qing and fell into a dilemma. Du found their shelter and accidentally discovered his lost important items, which also revealed a major secret.