The drama follows the lives of undercover police agents who must live a double life in secrecy. Not knowing when they can return to their normal life, they must also endure the constant fear of being uncovered by those they are assigned to infiltrate. However, this time their lives are put in danger when it is discovered that a mole lives within the police force.

In 2010, Cheuk Hoi secretly travels to Bangkok to conduct a drug operation with his undercover agents in Thailand. However the success of the mission had cost all of Cheuk Hoi's undercover agents to be killed in a sea of fire. Meanwhile back in Hong Kong, his good friend CIB Inspector Lai, is killed for unknown reasons, leaving Cheuk Hoi to take charge of a female amateur cop who is not yet officially an undercover. Cheuk Hoi knows that Lai Sir still has undercover agents whose identities have been deleted. To learn the truth about Lai Sir's death, Cheuk Hoi must work with Cheng Suk Mooi to uncover and secure the remaining of his late-friend's undercover agents while dealing with corrupt officials in the Police Force.

After completing his three-year undercover mission, Xue Jiaqiang returned to Hong Kong to open a foot bathing shop and continue to collect intelligence. Zhang Jizi, chief inspector of the criminal intelligence section, suspected that Zhuo Kai had been thrown into the underworld after he was released from prison and carried out comprehensive surveillance on him. Jia Qiang and Zhuo Kai were involved in the organ trafficking case. Together with female undercover Dou Yaxi and Security Bureau investigator Wei zuoreng, they worked together to smash the group involved. The Security Bureau and the newly established security intelligence agency were invited to join Xu Tiantang to take over the work of the Criminal Intelligence Division.