After a fatal shooting, plainclothes officer Him Law is assigned to protect the dull and thorny eyewitness Willie Wai. Initially they are resistant to each other, but the pair slowly learns to accept one another. Meanwhile, Willie's clothing company is discovered to be involved in illegal transactions. The company's senior executives, Lau Kong and his son Tsui Wing, are implicated in the scheme. Him joins forces with Willie and his lawyer sister (Kelly Cheung) to investigate. In the process, Him and Kelly develop a messy relationship, while Him's elder sister (Pinky Cheung) becomes foes with Willie. Fate pulls the four together, and a twenty-year old cold case is reopened...