This film was made in 1969, although the word 'doulos' doesn't appear in a modern edition of Le Petit Robert because the word is old-fashioned slang for 'indic', or 'indicateur', meaning 'nark', or to be more up-to-date: 'grass' (as in informant). In Jean-Pierre Melville's world anyone can be a grass and people (well, men, usually) have to tread carefully. Oddly, Maurice Faugel (Serge Reggiani) treads very carefully when entering the house of Gilbert Varove (René Lefèvre) and even wipes his shoes on the mat. What's odd about that is that Maurice will then shoot Gilbert dead after Gilbert (his friend, supposedly) has lent him the gun to use (but not hurt anyone) in a forthcoming robbery.
Maurice will believe that Silien (Jean-Paul Belmondo) is his friend until proved otherwise, but then when a robbery is interrupted by Inspector Salignari (Daniel Crohem), a friend of Silien's, and when Maurice's accomplice Rémy (Philippe Nahon) is killed and Maurice himself wounded (after killing Salignari) it becomes clear that Silien is in fact a grass. Or not.
This film is a convoluted mystery trip in which nothing will be revealed to the viewer until the last twenty minutes or so, and it's therefore no surprise that this is one of Quentin Tarentino's favourite films, particularly in regard to Reservoir Dogs. OK, this is néo-noir Melville again, but this film is not only an honour to the genre, but a profound work concerning the value of friendship, the value of money, conflict of interests, the nature of trust, outsiders, greed, desire, even the wish to have done with the lot and live in peace for the rest of your life. Just watch your back.