Great screenplay and some of the best actors the world has ever produced. Montand gives the concept of the 'lone wolf' police detective a whole new dimension of intensity and, most importantly, credibility.
When a typical Hollywood cop-heroe loses family, friends and pets to murder he is usually given his minute of grief. But when the sixty seconds are over, he pulls himself together, packs his gun and goes gleefully shooting up his enemies one by one.
Montand's Marc Ferrot, however, is really devastated - by his girlfriends murder, of course, but also by finding out that she had another lover.In his confusion and wrath he does not seek revenge but needs to keep going to find the real perpetrator of a crime where his fingerprints are all over the scene. Thus all his actions become unescapably logical. This is the main reason why this movie glues us to our seats but definetely not the only one.