Cheuk Hoi (Michael Miu) is a Chief inspector of Hong Kong’s Criminal Intelligence Bureau (CIB) police unit. He is a righteous supervisors that will resort to unethical tactics in order to clear the name of his subordinates that has gone missing and deemed by the police department to have strayed to the criminal side. When he finds his friend Hong Dou Hang, a fellow Chief inspector in the CIB unit thrown down a building mysteriously, he finds out from Hang's dying words that there is a mole in the Hong Kong police department. In order to protect the lives of five of his undercover agents Hang has deleted their files from the police data making them cease to exist as police officers. Hang tells Hoi to find each of these undercover agents, but dies before he could tell Hoi their names. Remembering what Hang told him and not trusting anyone in the department Hoi does not even tell his supervisors about Hang's dying words instead he investigates on his own to try to find who Hang's five undercover agents are.

With clues left behind by Hang, Hoi finds undercover agent Ting Siu Ka (Charmaine Sheh) who fronts as an owner of a foot message parlor, and assumes his role as her new supervisor. Siu Ka assignments under Hong sir was to only gather information but Hoi gives her the task to help him find the other four undercover agents. Siu Ka gets close to Sit Ka Keung (Raymond Lam), nicknamed Seed who is part of a gambling ring. He fronts as a ice deliveryman but is really in charge of collect unpaid gambling debts. Siu Ka previously butted heads with Seed and his partner Muk Sut but knows of their connections to a major triad organization.Meanwhile Kobe Lin Ho Kan (Sammy Shum), another undercover agent working for Hong sir, who is fronting as a crooked stock broker in order to go to prison and get close with triad boss Cham Foon Hei as his last assignment from Hong sir, but Foon Hei proves to be a more complicated person then Kobe expected. Not knowing if Foon Hei is joking or being serious, Kobe thought he had failed at his assignment. However upon Kobe's release from prison Foon Hei comes looking for him and offers Kobe to work for him. But in order for Kobe to follow through with his undercover assignment it had cost him his relationship with ICAC agent Yan Mok Sin Yan (Sharon Chan), who is not aware that Kobe is a undercover agent.

ICAC agent Yan who is investigating Hoi, due to every case he has been assigned to gets foiled, thinks he is an insider for the criminals who is tipping them off. With her tailing his every move it makes it complicated for him to meet up with the undercover agents. In order to be a step ahead of her he installs hearing devices in her home. What he finds out about her sister Katie Mok Sin Cing (Elena Kong) is more than he expected.

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