Philip Marlowe - The Big Sleep
"My, my, my! Such a lot of guns around town and so few brains!"
The first in the granddaddy of crime noir authors, Raymond Chandler's, Philip Marlowe series starred Humphrey Bogart as the aforementioned private dick, and Lauren Bacall as his is she/isn't she femme fatale.
While Marlowe is originally hired by a wealthy general who is aiming to clear his daughter's gambling debts, the involvement of his other daughter (Bacall), who tells Marlowe he's focusing his attentions in the wrong places, complicates matters somewhat.
Steely, timeless and a detective flick masterpiece that very few people, including the writer, fully understand (Chandler was later shocked to find he couldn't figure out who committed one particular murder).