Police officer Leung Siu Tong (Michael Tse), nicknamed "Laughing Brother", successfully completed his undercover mission and was given an important task in CIB. However, Chief Harry (Damian Lau) and his boss Jodie (Fala Chen) were suspicious of the death of his handler and found out that there was another hidden story in the investigation. After the CIB recruited new members, Laughing and Jodie became a pair of happy enemies due to their different training concepts. They won and lost against each other and refused to give in. In addition to taking care of his wife who was suffering from cancer, Harry had to face his ex-girlfriend Yuen Kwan-lam who had just been released from prison for 20 years because of him. Fortunately, his son Aidon was an outstanding dancer and was willing to help others, which made him feel at ease. In order to fight against Yee Hing, Laughing cultivated lame CoMichael (Bosco Wong) as an informant. The latter gradually gained importance in the fight between gang leader La Jiang and Willy, and broke away from Laughing's control. He rekindled his love with his former lawyer girlfriend Yao Keke (Tiffany Ann Tsui) and worked hard to get promoted. With the return of the gang leader Mo Yilie, the battle between undercover police and gangsters, and the struggle between black and white humanity intensifies...