Ha, nicknamed "Little Monster", is an elite member of the Criminal Investigation Division (CID) of the Royal Hong Kong Police Force. One time while arresting a criminal, Ha accidentally humiliates Chief Inspector Tin, whom is demoted as a result. Lui Kung (Lowell Lo), another CID officer, causes Tin's wife to give birth prematurely in a car accident while chasing criminals. Later, Ha and Kung were transferred to the Case Analysis Division, where they are supervised by Tin. Kung has a younger sister, Lui Lui, whom he cares for and treasures deeply and he does not allow anyone to pursue her. However, due to coincidences, Ha gets to know Lui and they develop a romantic relationship, much to the displeasure of Kung. Later, Ha and Kung work together to capture Lee Wai-fung (Charlie Cho), a former undercover cop who has gone rogue and becoming the biggest drug trafficker in Asia.