The Big Sleep is set in an unusually rainy Los Angeles. We open with Philip Marlowe, a private detective, arriving at the house of one General Sternwood, a potential client put forward by a friend of Marlowe's at the District Attorney's office. Sternwood, an ageing rogue who likes champagne mixed with brandy and oppressively hot rooms, wishes to engage Marlowe in a matter of blackmail. Sternwood's youngest daughter Carmen has apparently allowed herself to be photographed in compromising circumstances by a homosexual pornographer called Arthur Gwynn Geiger. Geiger is clearly a practised and professional blackmailer. The demand is sent via a polite little note which helpfully specifies the sum of money required and the arrangements for handing it over. For Marlowe, the assignment is a routine and simple one and he has no qualms about accepting it,
A complication arises, however, in the form of one Rusty Regan, retired bootlegger and missing husband of the other Sternwood daughter, Vivian. The matter of Rusty's disappearance is raised as an aside by the General. and in a negative fashion by Vivian. Marlowe's strongly developed sense of mystery is stimulated by this, and he resolves to find Rusty if at all possible.
Marlowe stakes out Geiger's pornography shop, which is disguised as a rare book shop. From here he is able to identify and follow Geiger to his home, which he stakes out in turn. During this time, Carmen Sternwood arrives and goes into the house. There is a flash and a scream and Marlowe approaches the house. Whilst in the process of making an entry, Marlowe hears three shots and the footsteps of someone making a rapid exit. Upon entering the house, he discovers that Carmen is in the process of being photographed yet again. She is near senseless with drugs and completely naked. Geiger's body is on the floor nearby.The photographs, which Marlowe had intended to either purchase or steal, are gone. Carmen is too intoxicated to be of any help so Marlowe takes her home.
Marlowe is woken later by Bernie Ohls, his friend at the DA's office, who informs him that the Sternwood's chauffeur has been found dead in the family car, Indications are that Taylor, the chauffeur, had been infatuated with Carmen and had, for this reason, shot Geiger for taking nude pictures of his lady love. He had then either been killed by person or persons unknown, or had committed suicide.
Meanwhile, two of Geiger's employees - Joe Brody and Agnes Lozelle - are plotting a takeover of the pornography business. Their planning is crude and obvious and. as a result, Marlowe ends up tracking Brody back to his apartment where he and Agnes have been conspiring. Marlowe shakes them down, discovering in the process that the pictures in question are in Brody's possession, and that Brody has already used them to blackmail Vivian, Carmen's sister. the meeting is a farce. They are initially interrupted by a drugged, gun toting Carmen, demanding the return of her pictures. Marlowe disarms her and keeps the gun. Marlowe is on the verge of recovering the pictures himself when a man called Carol Lundgren, who turns out to be Geiger's bereaved homosexual lover, storms the apartment and murders Brody in the mistaken belief that he was Geiger's killer.Both Lundgren and Agnes are arrested, with Agnes subsequently released. Marlowe obtains possession of the pictures and renders them safe. The death toll has resulted in some media attention, but the reporting is so inaccurate as to be meaningless.
At this point, Marlowe's work is done. Marlowe, however, does not see it that way. He suspects that the absent Rusty Regan may have been involved in the blackmail. He suspects also that General Sternwood would be very interested to confirm or disprove this. On top of this, Marlowe has become curious about the circumstances of Rusty's disappearance, as well as the unsolicited reactions he observes whenever it is erroneously assumed that he has been hired to find Rusty.
At this point Marlowe becomes aware that he is being somewhat inexpertly followed. He confronts the tail. who turns out to be a man called Harry Jones. Harry, one of many mistakenly convinced that Marlowe's principle talk is to locate Rusty, wishes to sell information he has regarding the location of one Mona Mars nee Grant. Mona is the wife the notorious racketeer Eddie Mars, and the popular rumour has it that Rusty eloped with her. Harry, however, claims that Mona is being kept out of sight by her husband Eddie and that he knows where she is being kept. Harry, who has partnered up with Agnes, is keen to sell what he knows for a sum sufficient to get both of them comfortably out of town. Marlowe is keen to purchase Harry's information, but is forestalled by Lash Canino. Eddie Mars' murderous enforcer, who kills Harry and attempts to kill Agnes.
In spite of Canino's efforts, Marlowe has discovered Mona's location. He drives there on yet another rainy night. At a mechanic's garage outside town Marlowe encounters Canino and a confederate. They fight and Marlowe is bound and left in the house where Mona is being kept. Mona is there also, and is fully aware that Canino intends to murder Marlowe as soon as he has time to do so. Marlowe uses her good nature to persuade her to release him, He then waits in ambush for Canino and friend. They arrive, there is a gunfight and, with Mona's help, Marlowe kills both men.
Now that the pictures are safe and the best lead on Rusty has been confirmed as a dead end, Marlowe plans a final trip to the Sternwood house to close out the job. While there, he remembers to return Carmen's pistol after having confiscated it earlier at Brody's apartment. Carmen pesters him into teaching her how to shoot, so Marlowe agrees. Aware, however, of Rusty's probable fate, he loads the pistol with blanks before following Carmen down to an isolated corner of the family's old oil field. Upon confronting Carmen, she attempts to murder him but fails due to Marlowe's foresight. Marlowe takes Carmen back to Vivian. He agrees to keep her murder of Rusty a secret from both the police and from General Sternwood on the condition that Vivian places Carmen under psychiatric care.
The novel ends on a note of sadness, with Marlowe thinking of Rusty sleeping 'the big sleep', and of Mona, with whom he has fallen fruitlessly in love,