The Big Sleep is a 1946 detective movie starring Humphrey Bogart, Lauren Bacall, and a whole bunch of other people who aren't quite as well known anymore.
Bogart plays Philip Marlowe, hired by a rich old dude named Sternwood (Charles Waldron) to find out who's blackmailing his youngest daughter, Carmen (Martha Vickers). Carmen, as it happens, is flighty and has trouble with gambling, drugs and liquor. Her older sister, Vivian (Bacall), has her own issues with gambling, and maybe the blackmailer knows her, too?
The movie, from there, spirals into an absolutely labyrinthine mess of subplots, characters, murder, crime and general noir-ishness. The knots don't get untangled so much as more tightly wound around Marlowe. If you want to know the plots, go read the wiki or read the novel, because no way am I qualified to explain it after one viewing.
What you need to know is that it's really well scripted and acted, and although it's busy as hell, it doesn't feel confusing (even though it is). It reminded me in some ways of L.A. Confidential (I suspect the screenwriters of that movie were, kind of successfully, looking to emulate Chandler). And Bogey and Bacall have some fun dialog and chemistry like whoa, so it's very much worth your time.
My grade: A
Rewatch value: Medium
Next up: Ghost