Set in Hong Kong, China, four years after the events in Line Walker, Sit Ka-keung (Raymond Lam) continues to work as a Hong Kong Police Criminal Intelligence Bureau (CIB) undercover officer, while Cheuk Hoi (Michael Miu) opens a coffee shop after being released from prison. CIB Chief Inspector Cheung Kei-gee (Mandy Wong) suspects Check Hoi of joining the triad and investigates him. The Security Bureau establishes Security Intelligence Agency (SIA), a new intelligence department. The principal investigator of SIA, Wai Chok-wing (Kenneth Ma) invites Tsui Tin-tong (Benjamin Yuen) to join SIA. Chum Foon-hei (Benz Hui), who was thought to be killed by Bao Seed, returns to Hong Kong to track down the leader of an international crime organization, Eternity. The true identity of Wai Choi-wing is later revealed to be Ngai Tak-lai, the younger paternal half-brother of Victor Ngai Tak-shun...