[5][6] Koo and Lam play ICAC investigators who battle against notorious drug lord Crippled Ho (Leung) and corrupt police detective Chui Lok (Ng).

As the kings of the corruption empire, chief Chinese detective Chui Lok (Francis Ng) and triad leader/drug lord Crippled Ho (Tony Leung Ka-fai) have colluded for over ten years and amassed over a trillion dollars.

Lawyer Hank Chan (Louis Koo) has always despised corruption and believes that if citizens fall into poverty, their rights and privileges would be exploited.

Pak is impressed by Chan's righteous personality and invites him to join the governor's newly established Independent Commission Against Corruption.

Chan and Pak select a group of elites to be the ICAC's first batch of investigators, with their main targets being Ho and Tsui.

Principal photography for Once Upon a Time in Hong Kong began on 28 August 2019 where a production commencement ceremony was held with director Wong Jing and stars Louis Koo and Gordon Lam present.