The movie is a French neo-noir that comes from 1962. The setting is the Parisian underworld. The dodgy nightclub owner is Michel Piccoli. The snitch paranoid that a just released prisoner is out to get him is Jean-Paul Belmondo. The title refers to a type of hat, the kind worn by the informer, hence why "doulos" becomes slang for a criminal canary. The cinematography is black and white. Almost everybody smokes. And the director is Jean-Pierre Melville. Need we say more?