Based on a mystery writer's novel which I've not read, the film succeeds courtesy of some great acting. So often a mystery is put together with clues and red herrings, and then at the end you fold a puzzle piece and voila it fits. I think it appeals to the clockwork mind, and as their is often crime involved, there is a sense of some extracurricular guidance towards justice.
While this film has that, and action moves like car chases, pistol whipping and a lengthy running script (the kind Tom Cruise would demand be inserted in a film of his), Francois Cluzet holds the center of the film in more gentle hands. He's a good guy, even when mixed up with the bad guys, he brings out their inherent benevolence. You go, Bruno!
Films like this hinge upon believing that bad moves are really the best moves one can take, the story and acting help sell that here in my opinion. In real life, I think in most cases perhaps not as intricate as this, I would Tell Some One, and likely an authority. Granted plenty of films from America North and South, make clear the dangers of corrupt cops and who watches the watchers. Maybe in Europe, things could be different.
Again for the genre, experts might not enjoy this (or pick it apart for a variety of reasons). But I was able to just enjoy the people on screen foremost (always helps to have Nathalie Baye involved), and then the pacing and plot.
Nice to see the director hatched the script and put himself in the mix on screen as well. Recommend you watch it, and then figure out the right someone to tell to see it next.
Also excellent music throughout, especially that closing track.