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, TOU FA (Selena Lee). years later, the aging Nam Tin orders three of his most-skilled apprentices – MA CHUEN-KUNG (Power Chan), NGAU DAI-LIK (Pierre Ngo), and YEUNG MAU (Rosina Lin) – 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. Sing learns that the opposing Jurchen kingdom has placed a mole within the Tin Kei Agency and his good friend, HUNG SUP-KAU (Raymond Cho) is exploited by the enemy. Cheung Sing does not know whom to trust. The line between friend and foe continues to be blurry.