The Big Sleep is the story of Philip Marlowe, a hard-boiled private investigator called in by a wealthy client to deal with blackmail attempts leveled against the client. Marlowe uncovers a nest of familial dysfunction and double-crossing during the course of his investigation. He survives many close calls with death as he exposes the full extent of the labyrinthine plots occurring within his client's family. He is able to solve the case through guile and perseverance.
Private investigator Philip Marlowe is called to the home of the wealthy and elderly General Sternwood, in the month of October. He wants Marlowe to deal with an attempt by a bookseller named Arthur Geiger to blackmail his wild young daughter, Carmen. She had previously been blackmailed by a man named Joe Brody. Sternwood mentions his other, older daughter Vivian is in a loveless marriage with a man named Rusty Regan, who has disappeared. On Marlowe's way out, Vivian wonders if he was hired to find Regan, but Marlowe will not say.
Marlowe investigates Geiger's bookstore and meets Agnes, the clerk. He determines the store is a pornography lending library. He follows Geiger home, stakes out his house, and sees Carmen Sternwood enter. Later, he hears a scream followed by gunshots and two cars speeding away. He rushes in to find Geiger dead and Carmen drugged and naked, in front of an empty camera. He takes her home but when he returns, Geiger's body is gone. He quickly leaves. The next day, the police call him and let him know the Sternwoods's car was found driven off a pier, with their chauffeur dead inside. It appears that he was hit on the head before the car entered the water. The police also ask if Marlowe is looking for Regan.
Marlowe stakes out the bookstore and sees its inventory being moved to Joe Brody's home. Vivian comes to his office and says Carmen is being blackmailed with the nude photos from the previous night. She also mentions gambling at the casino of Eddie Mars and volunteers that Eddie's wife, Mona, ran off with Rusty. Marlowe revisits Geiger's house and finds Carmen trying to get in. They look for the photos, but she plays dumb about the night before. Eddie Mars suddenly enters; he says he is Geiger's landlord and is looking for him. Mars demands to know why Marlowe is there; Marlowe takes no notice and states he is no threat to Mars.
Marlowe goes to Brody's home and finds him with Agnes, the bookstore clerk. He tells them he knows they are taking over the lending library and blackmailing Carmen with the nude photos. Carmen forces her way in with a gun and demands the photos, but Marlowe takes her gun and makes her leave. Marlowe interrogates Brody further and pieces together the story: Geiger was blackmailing Carmen; the family driver, Owen Taylor, didn't like it, so he sneaked in and killed Geiger, then took the film of Carmen. Brody was staking out the house too and pursued the driver, knocked him out, stole the film, and possibly pushed the car off the pier. Suddenly the doorbell rings and Brody is shot dead; Marlowe gives chase and catches Geiger's male lover, who shot Brody thinking he killed Geiger. He had also hidden Geiger's body, so he could remove his own belongings before the police got wind of the murder.
The case is over, but Marlowe is nagged by Regan's disappearance. The police accept that he simply ran off with Mona Mars, since she is also missing, and since Eddie Mars wouldn't risk committing a murder in which he would be the obvious suspect. Mars calls Marlowe to his casino and seems to be nonchalant about everything. Vivian is also there, and Marlowe senses something between her and Mars. He drives her home and she tries to seduce him, but he rejects her advances. When he gets home, he finds Carmen has sneaked into his bed, and he rejects her, too.
A man named Harry Jones, who is Agnes's new partner, approaches Marlowe and offers to sell him the location of Mona Mars. Marlowe plans to meet him later, but Mars's henchman Canino is suspicious of Jones and Agnes's intentions and kills Jones first. Marlowe manages to meet Agnes anyway and receives the information. He goes to the location in Realito, a repair shop with a home in back, but Canino, with the help of Art Huck, the garage man, jumps him and knocks him out. When he awakens, he is tied up, and Mona Mars is there with him. She says she hasn't seen Rusty in months; she only hid out to help Eddie and insists he didn't kill Rusty. She frees him, and he shoots and kills Canino.
The next day, Marlowe visits General Sternwood, who remains curious about Rusty's whereabouts. On the way out, Marlowe returns Carmen's gun to her, and she asks him to teach her how to shoot. They go to an abandoned field, where she tries to kill him, but he has loaded the gun with blanks and merely laughs at her; the shock causes Carmen to have an epileptic seizure. Marlowe brings her back and tells Vivian he has guessed the truth: Carmen came on to Rusty and he spurned her, so she killed him. Eddie Mars, who had been backing Geiger, helped Vivian conceal it by first helping to dispose of Rusty's body, inventing a story about his wife running off with Rusty and then blackmailing her himself. Vivian says she did it to keep it all from her father so he wouldn't despise his own daughters, and promises to have Carmen institutionalized.
With the case now over, Marlowe goes to a local bar and orders several double Scotches. While drinking, he begins to think about Mona "Silver-Wig" Mars. He never sees her again.