This is the second film (the first was done in 1946) based on Raymond Chandler's 1939 detective novel. Michael Winner, directing from his own screenplay, has shifted the locale of the story from Los Angeles to London. Otherwise the movie sticks close to the text.
The Big Sleep opens with Philip Marlowe approaching by car the mansion of a prospective client whom he describes in a voice-over narration as being very rich. General Sternwood is a decrepit, retired military man who is troubled by a recent blackmail attempt. Marlowe agrees to take the case. He soon finds himself involved in the amoral world of the rich and the decadent realms of the lost and lonely. Two of the most confused and confusing characters are the General's daughters: Charlotte who has been abandoned by her husband (everyone wants to know where he his) and Camilla, a slightly batty girl.
The major flaw in this cinematic interpretation of The Big Sleep is that Chandler was always more interested in people than in the mechanics of his story's development. The writer/director, relying on an overuse of flashbacks, emphasizes the latter to the exclusion of the former. The second drawback is the inadequate performances put in by Sarah Miles as Charlotte, Candy Clark as Camilla, and Oliver Reed as an underworld figure. They all overact.
However, despite these deficiencies, Robert Mitchum's wry and worldly depiction of Marlowe carries the movie. Here is a hero who believes in the old verities as William Faulkner called them "love and honor and pity and pride and compassion and sacrifice." Nowadays it's hard to find a decent man on the street or in the theatre. To see one on the screen in an inspiration indeed.