Ding Xiaojia (played by Charmaine Sheh), who came from the bottom of Hong Kong society, returned to the mixed Portland Street as an undercover agent after passing the police exam to collect intelligence for CID Inspector Kang Daoxing (played by Au Shui-wai). Unexpectedly, Daoxing was killed in the process of tracking down the criminal facts of the triad leader You Dafu. Before his death, he deleted the records of five undercover agents including Xiaojia to prevent the identities of his undercover personnel from being exposed. After that, Daoxing's colleague, Chief Inspector Zhuo Kai (played by Michael Miu) of the Criminal Intelligence Bureau, contacted Xiaojia, hoping that he could help find the other four undercover agents and obtain criminal evidence of the peripheral gambling group. After that, Xiaojia and the gambling group's capable man Xue Jiaqiang (played by Raymond Lam) intersected, and thus fell into a series of confrontations and fights between the police and criminal gangs and gang organizations, thus staging a stunning undercover legend...