After losing both parents at a young age, Lai Yat-tsau (Bosco Wong) becomes a parent figure for his three younger sisters. His mother’s death and his father’s fall to drug addiction turned Yat-tsau into a very quick-tempered and chauvinistic man.

Finding him too impulsive to be part of the Criminal Investigative Department, Yat-tsau’s superior transfers him to the K-9 unit to work with police dogs, hoping that the strict training would better shape his character. Yat-tsau is assigned to work with a K-9 called Brother, but Brother does not take a liking to him.

During this process, Yat-tsau steps on the wrong end of veterinarian Ma Chi-ho (Linda Chung), and the two strong personalities clash. But Chi-ho, who needs Yat-tsau’s help in mending her estranged relationship with younger brother Ma Chi-chung (Oscar Leung), volunteers to aide Yat-tsau in improving his relationship with Brother. Through Brother, Yat-tsau and Chi-ho develop an interesting partnership and chemistry.

With Chi-ho’s help, Yat-tsau and Brother Bond. Brother also helps Yat-tsau track down the drug dealer Lui Kam-dong (Jerry Ku). Just as he is closer to solving the case, Yat-tsau’s partner and close friend Fai Chung-lim (Raymond Wong) breaks an unwritten rule in order to keep a secret, putting a dent on their brotherhood. This also gets Chung-lim’s girlfriend and Yat-tsau’s younger sister, Jil (Tracy Chu), into a near-fatal disaster.

Kam-dong returns for revenge, and uses his younger twin brother Kam-bo, who has been working as an informant for the cops, to concoct a trap. Yat-tsau, Chi-ho, and Brother team up to save their friends.