Noir, to me, is about doom. The noir hero is a hero because he struggles heroically, but the struggle is always a doomed one, and usually because of his own tragic failings. The sins of the past can never be escaped, they will always come back in some form to destroy whatever you might have built- that's the theme of film noir.

My favorite of all film noir movies is 1947's “Out of the Past,” also known as “Build My Gallows High.” When the main character- a private detective played by Robert Mitchum- has to choose between loyalty to his gangster client and desire for Jane Greer's femme fatale character, he picks the girl. His lapse in judgment proves disastrous, but he does manage to get away- for a time. Then one of the gangsters recognizes him pumping gas under an assumed name in an out-of-the-way town in the desert, and it all starts up again.

The detective's doomed attempts to escape the spiderweb are complicated by his “good girl” girlfriend, who doesn't know about his disreputable past or her sinister yet compelling rival. Much of the story is told as a flashback, in which he attempts to explain how he got into such a mess. In the end, of course, he has to go out and face his destiny, and even though you root for him you know it can only really end one way. Some movies are noir classics despite having one foot in the conventions of the genre and one foot out. “Out of the Past” is pure film noir.