The Tin Kei Agency of the Northern Song, an investigation agency specializing in solving strange cases in the kingdom, is headed by the wise Fung Nam Tin (Lau Kong), who leads a group of extremely well-trained constables. His favorite apprentice, Chu Cheung Sing (Kenneth Ma), is wisdom and courage embodied in one, but Nam Tin and Cheung Sing end their partnership when they fail to come to a consensus on how to solve a particular case. Cheung Sing ends up leaving the agency to live a simple and peaceful life with his wife, To Fa (Selena Li).Ten years later, the aging Nam Tin orders three of his most-skilled apprentices – Ma Chuen Kung (Power Chan), Ngau Tai Lik (Pierre Ngo), and Yeung Mau (Lin Xiawei) – to reach out to Cheung Sing and convince him to return. Cheung Sing agrees to work with the agency again, but his personality often clashes with his three partners, causing the four to frequently fight. Meanwhile, Cheung Sing is separated from his wife, but his feelings become conflicted when he ends up meeting a prostitute who looks exactly like her.