"Line Walker" is another one of the long line of movies with cops, crooks and moles. In this case, it is based (continues) the story of a TV series where a series of police undercover found their files deleted and on the hide, no one knowing they were actually cops. The intro to the movie tells us that all the cops where found, but we soon discover that there was one left behind.
From now on we will have the typical game of cat and mouse, where we are supposed to find who is a mole, who is a policeman, who is what and why and for what purpose. All this has been done before ("Infernal Affairs" is an amazing example), but in "Line Walker" it falls into a convoluted plot, that jumps from one place to the other (it feels as if some parts of the movie have been cut) and can't really hold the viewer's attention.
The actors all do a good job. Louis Koo, Francis Ng, and all the others are as charismatic (and blank sometimes) as ever, and bring the story to life in its low moments. The action scenes are fine, even though a little bit messy sometimes and fall into the yes-we-can't-really-shoot-what-can-we-do problem.
All in all, it is an average movie, that will be better enjoyed (probably) by fans of the TV series.