Le doulos = hat = police informant.
For Jean-Pierre Melville, this is a surprisingly fast-moving story based on the distrust between criminals, police and police informants. It turns into a fine whodunnit so it helps to keep you wits about you.
This is the second Jean-Paul Belmondo performance I've seen in a Melville films and they were both outstanding. He is more subdued in these films than is typical for Jean-Paul, yet he has more life and dimensions than Alain Delon who would become Melville's choice for later films. Lesser-known Serge Reggiani is equally splendid in the other major role of the film (he is the focus of the first half of the film).
This is a B&W film with 60s clothing styles for the women and 50s style for the men (trenchcoat and hats). Not as distinctive as the other films I've seen of Melville's, but, because of its storytelling and pacing, it's one I can recommend to people who wouldn't tolerate his slower-paced films. In other words, this one is just plain fun.
This DVD includes commentary for selected scenes and an on-camera lecture by French film expert, Ginette Vincendeau, and comments by Assistant Director to Melville, German-born Volker Schlondorff. Both are extremely fluent in English and offer fascinating information.