It's only somewhat appropriate that Jing Wong ended up directing a Once Upon a Time film about Hong Kong. With a slew of actors that helped boost his career (and visa versa of course), he made a pretty decent but ultimately generic crime epic that fits well into his oeuvre but does very little to stand out amongst a crowd of literally hundreds of films.
In the early 70s, crime in Hong Kong is down, only it was due to the police being corrupt and taking bribes, working together with the criminals. The ICAC is founded to battle this corruption. It's a steep hill to climb, as nobody trusts the new anti-crime unit, but thanks to some upstanding detectives they start to win the trust of the Hong Kong public.
Some very high-profile names here, but it's Francis Ng who stands out and grants his character some extra depth. The plot is pretty simple, the cinematography decent but nothing too wild, and the ending predictable (certainly with China calling the shots in Hong Kong nowadays). Decent crime fluff, but we all know they can do a lot better in this niche.