The plot: A random letter by convict on death row lands Pil-Jae's attention. Initially Pil-Jae investigates to take revenge against his former colleague (who framed him as a dirty cop) but soon he realizes that he's kicked a hornet's nest. The queen of the nest is the Madam, who is the head of Dahai corporation (probably based on Hyundai), and she's a nasty business as women usually are in films like this.
There are nods to genre classics such as "Chinatown" and the audience of Korean cinema will recognize the current trend of "Rich villains vs Rough heroes" also seen in (and done much better) "Veteran" and "Inside Men". And it is with these comparisons, the film's problems become visible. This film does not offer anything other than simple plot progression. There's setup to intrigue, there's problem solving in the middle and then comes the solution. While "Veteran" had some memorable action sequences and "Inside Men" offered gritty and dirty look at high class society in Korea, this film offers neither and wastes its characters in obligatory clichés.
Cliché #1: Dong-Il Sung's character. The usual comedic relief sidekick (whose position would actually make him Kim's boss) who occasionally tags along and requires some rescuing later.
Cliché #2: The victims. Of course they are completely innocent. Of course they make all the wrong choices that make problem solving more complicated than it already was.
Cliché #3: The climax. After all that running around and getting beaten up, the solution to the crisis at hand relies on the old "Aha! I was recording this conversation all along!"
All this would be forgivable had "Proof Of Innocence" been at least entertaining. Sprinkling some stunt sequences here and there, giving it more shock value would have helped out a lot. Alas, director Kwon cannot escape his own below average directing skills. At the end of the day, not taking any risks in trying something fresh has created another generic product in already rotting pit of Korean cinema.