After his mothers death & drug addicted fathers departure, officer Lai Yat Chow (Bosco Wong) single-handedly raised his three younger sisters. The eldest Suk Chu (Kayi Cheung) is a kind kindergarten superintendent, middle sister Jil (Tracy Chu) is a materialistic salesperson whos dating Yat Chows rich partner Fai Chung Lim (Raymond Wong), & the youngest Fanny (Angel Chiang) is married & unemployed.
During a drug bust, Yat Chow is attacked by the gangs guard dog Bah Da. To prove that Bah Da is not dangerous, veterinarian Ma Chi Ho (Linda Chung) visits Yat Chow & asks about the details of the incident. However, her bluntness offends him & the two get off to a bad start. Chi Ho soon discovers Bah Da is able to detect illegal drugs so she recommends it to be trained as a police dog. On the other h&, Yat Chow is transferred to the Police Dog Unit & appointed the handler of Bah Da. With the help of Chi Ho, Bah Da & Yat Chow develop a mutual understanding & solve various cases together.