Line Walker: The Prelude - Season 1

In 2010, Cheuk Hoi secretly travels to Bangkok to conduct a drug operation with his undercover agents in Thailand. As they finish their mission and are about to leave Thailand, an explosion causes all of Cheuk Hoi's undercover agents to die in a sea of fire. At the same time, Cheuk Hoi's good friend, CIB Inspector Lai Sir, is killed for unknown reasons, leaving Cheuk Hoi in charge of a female 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 find all of the undercovers. He and Suk Mui begin to search for the undercover agents.