The movie version of the actor's final ending didn't understand much. What is undercover? What are the real identities of Louis Koo and Nick Cheung?

Allen, played by Nick Cheung, is a real undercover. He didn't contact Mr. Q. On the contrary, Louis Koo, who is not a policeman, pretends to be undercover. But in the end, this plot did not clearly explain why Louis Koo pretended to be undercover, and he had no reason to do so. If it is beyond Nick Cheung, the fraternity promoted in the whole movie is a big joke.

Obviously, the writer did this only for the wonderful reversal of the plot, probably because he felt that the audience would not think again after being shocked by the plot at this moment.