After a criminal's shooting leaves 4 PTU officers' lives altered, years later they reunite to fight a drug lord, with one officer now a gangster himself, leading to fierce battles to restore order.

Rewind to 2019, and jewelry heist hardened criminal Chan Cho-yiu's (Benjamin Yuen) cold-blooded shot changes four PTU officers' destinies. Fast-forward five years, and among the four police officers, only Kwan Chi-hin (Joel Chan) is still staying with PTU. Tsui Kwok-yin (Owen Cheung) has quit his police job and become a gangster. Chi-hin and his partner So Kwok-wai (Jonathan Cheung) oversee rookie PTU officers. They assist Anti-Triad Unit Inspector Pang Wing-kei (Rebecca Zhu) in apprehending drug dealers. They again astonishingly run into Cho-yiu. Cho-yiu is determined to get even with the notorious drug lord West Kowloon King (Hugo Wong), aka Ruthless. Cho-yiu then takes revenge on him, but West Kowloon remains anarchic as bad people come from everywhere. To restore law and order, KW Anti-Triad Unit and PTU deal with the gangsters by engaging them in fierce battles. And there is no end in sight.