A diligent and skilled Hong Kong police officer is assigned to a case that requires him to go undercover to infiltrate a drug trafficking syndicate. The officer successfully poses as a petty thug and earns the trust of a notorious crime lord. His aim is to gather intelligence about the criminal organization, find evidence of their illegal activities, and ultimately bring down the drug kingpin and his empire. To maintain his cover, he is sent to a mainland Chinese prison where he must survive among hardened criminals.

While undercover, the officer partners with a tough and resourceful female mainland police inspector who joins him in his mission. Disguised as his sister, she aids him in navigating through dangerous territory as they get closer to the core of the criminal organization. The two officers rely on their combat skills and quick wits as they work together in a high-stakes environment, knowing that one wrong move could blow their cover and put their lives at risk.