The film revolves around the exploits of a detective agency in Hong Kong called Mannix Private Detective Agency. It is headed by private detective Wong Yeuk Sze (Michael Hui) with his emotionally drained assistant Puffy (Ricky Hui). Meanwhile, Lee Kwok Kit (Samuel Hui), a kung fu expert, who works at a Vitasoy plant factory and spends most of the time doing kung fu tricks to impress a girl, ultimately loses his job. Seeking to find another line of work, Lee attempts to joins Wong's detective agency. Despite Lee's impression with his kung fu talent which involves his snatching trick, Wong was not impressed. Then, as it appears that Lee would not get the job, Wong discovers that his wallet was missing and was presumed stolen by one bystander who bumped into them. Lee intercepts him and recovers the wallet, thus impressed Wong to hire him for the job.

A young man suspects his father's suicide was actually a murder committed by gangsters. Using his expertise in martial arts, he gets himself hired by the gangsters who he suspects are responsible for his father's death.