Private detective Philip Marlowe is deep into an unproductive missing person investigation when he unexpectedly crosses paths with a notorious felon named Moose Malloy. Bursting into a nightclub known as Florian's, Moose is searching for his ex-girlfriend Velma Valento. However, the club's ownership has changed hands, leaving no one familiar with her. Tragically, during his desperate search, Moose kills the club's black owner and manages to escape. This murder case is handed over to Lt. Nulty, a Bay City police detective who shows little interest in the death of a black man. Despite Marlowe's suggestions to track down Moose's girlfriend, Nulty prefers to let Marlowe tackle the laborious ground work while banking on recognizing Moose based on his significant size and loud wardrobe. Resolute, Marlowe decides to continue his search.
His investigation brings him to Mrs. Jessie Florian, the widow of the previous nightclub proprietor, where he discreetly offers her some bourbon to loosen her tongue. Mrs. Florian recalls Moose's girlfriend and informs Marlowe that she has passed away. Before heading out, Marlowe manages to secure a photograph signed "Always yours, Velma Valento" to aid in his investigation. Just as he is gaining traction, Marlowe receives a call from Lindsay Marriott, who claims his friend has been robbed and asks for Marlowe's assistance in delivering ransom money for stolen jewelry. Unfortunately, later that night in a deserted canyon, Marlowe is unexpectedly struck from behind. Upon regaining consciousness, he discovers that Marriott has been murdered. Thankfully, a passerby named Anne Riordan finds him and takes him back to safety.
Lt. Randall, an astute yet honorable Los Angeles police officer, begins to question the veracity of Marlowe's story regarding Marriott's murder. At his office, Anne discloses her background as a policeman's daughter from Bay City, who has an interest in the local crime scene. Her father was dishonorably discharged by corrupt officers within the Bay City police force. She informs Marlowe that the stolen necklace belongs to Mrs. Lewin Lockridge Grayle, a stunning blonde who is the young spouse of a wealthy Bay City magnate. Mrs. Grayle's past is shrouded in secrecy; she married her husband in Europe while using an alias to protect her true identity. Anne expresses a desire to have her hire Marlowe to help recover the necklace.
Marlowe examines some marijuana cigarettes that were discovered on Marriott's body and stumbles upon a card belonging to a psychic named Jules Amthor. Curious, he arranges to visit Amthor's office for a consultation. However, following a hunch, he investigates Mrs. Florian's home and uncovers that Marriott had a trust deed on the property, giving him leverage over her. During his next conversation with Mrs. Florian, she divulges that she once worked as a servant for Marriott's family, leading Marlowe to suspect she was involved in blackmailing him.
Marlowe soon finds himself in the charming presence of Mrs. Grayle, who becomes infatuated with him and hires him. This advantageous relationship provides Marlowe with the perfect cover to continue his inquiries into the two murders. They plan to meet again at a club run by local criminal Laird Brunette, situated near where Marriott lost his life.
At Amthor's office, Marlowe confronts him about his connections to Marriott and the drug scene. Amthor, sensing trouble, enlists Bay City detectives to arrest Marlowe by falsely accusing him of blackmail. Instead of taking him to jail, the detectives render him unconscious and confine him in a private hospital overseen by Dr. Sonderborg, a drug dealer intent on keeping Marlowe subdued with injections. Marlowe eventually escapes, only to see Moose in another room while making his exit. He later discusses the case with Randall, who grows increasingly frustrated with Marlowe's relentless pursuit of answers. Both men come to suspect that Marriott was blackmailing wealthy women in cahoots with Amthor, and they rush back to Mrs. Florian's place, only to discover her murdered, seemingly a victim of Moose's wrath.
Due to the shady involvement of the Bay City police, Marlowe seeks out the corrupt Bay City police chief, John Wax, who dismissively brushes him off. That is until Marlowe mentions Mrs. Grayle's hiring. This gives Marlowe a lead, and he is informed that Moose might be hiding out on a gambling boat run by Brunette beyond the three-mile federal limit, where the corrupt city officials operate. With assistance from honest cop Red Norgaard, who was also wrongfully dismissed by Bay City, Marlowe manages to sneak aboard the gambling boat. There, despite being caught by Brunette, he persuades him to relay a message to Moose through the criminal network.
To maintain appearances, Marlowe calls Mrs. Grayle under the pretense of arranging their date at his apartment. However, responding to Marlowe's message, Moose arrives first, promptly hiding as Mrs. Grayle enters. During their conversation, Marlowe confronts her, revealing she is Velma and had manipulated Marriott into controlling Mrs. Florian. It becomes clear that Marriott was a target when Velma, viewing him as a liability that could expose her past, orchestrated his death. As the truth unravels and tensions peak, Moose emerges to confront Velma, leading to a dramatic confrontation where she fatally shoots him before escaping.
Ultimately, the deceit of Amthor, Sonderborg, and the corrupt police officers is brought to light, and Red finds his job reinstated. Velma, however, remains at large, and when she is finally located in Baltimore, she tragically kills a detective who recognizes her and takes her own life when cornered.
