Returning home from a late-night grocery run for cat food, Philip Marlowe (portrayed by Elliott Gould) unexpectedly finds his close friend, Terry Lennox (played by Henry Gibson), in his Los Angeles apartment. Lennox, in a state of distress, urgently requests Marlowe to drive him to Tijuana, prompting Marlowe to notice the scratches marring his friend's face.
Upon returning from Mexico, Marlowe is intercepted by two police detectives who begin to question him about Lennox. When Marlowe refuses to answer their inquiries, he is placed under arrest. Through the course of his interrogation, Marlowe learns that Lennox stands accused of the murder of his wife, Sylvia. However, after three grueling days in jail, he is released upon the shocking revelation that Lennox has committed suicide in Mexico. Despite this news, Marlowe remains skeptical, convinced that Lennox could not have killed either Sylvia or himself.
In a twist of fate, Eileen Wade (played by Nina van Pallandt) enlists Marlowe's help to track down her missing husband, Roger Wade (portrayed by Stephen Coit), an alcoholic novelist struggling with writer's block. Roger's deeply ingrained machismo, reminiscent of Hemingway, leads him into self-destructive behavior and long absences from their home in Malibu Colony. Marlowe's investigation takes him to a private detox clinic for affluent addicts, where he finds Roger and brings him back. It's during this visit that he uncovers the Wades' social ties to the Lennoxes, which piques his curiosity about the true nature of their connection.
The plot thickens when Marlowe encounters gangster Marty Augustine (played by Sterling Hayden), who demands the return of the $355,000 that Lennox was sent to deliver to Mexico City. In a brutal show of force, Augustine threatens Marlowe by inflicting harm on his girlfriend. Returning to the Wades, Marlowe learns that Augustine actually owes Roger a sum of $50,000.
Marlowe's journey leads him back to Mexico, where local officials affirm the details surrounding Lennox's death. Following this, he returns to a chaotic beach party at the Wade household, where the atmosphere shifts when Dr. Verringer arrives, seeking payment from Roger for his clinic fees. Amidst the mounting tension, Roger abruptly walks into the ocean, leading both Eileen and Marlowe to chase after him. Tragically, he disappears and is soon presumed drowned. Eileen reflects on Roger’s affair with Sylvia and suggests that his jealousy might have driven him to murder her, something the police are already aware of.
In a final twist, Marlowe receives a farewell letter signed "Terry," accompanied by a mysterious $5,000 bill. This leads him to a meeting with Augustine, where an unexpected incident with Marlowe's wallet uncovers the bill, prompting Augustine to return it, believing it to be linked to his stolen money. After this encounter, Marlowe spots Eileen departing in her open-topped Mercedes-Benz 450SL. In an attempt to reach her, an oncoming vehicle strikes him, landing Marlowe in the hospital. Here, he receives a curious gift: a miniature harmonica from a heavily bandaged fellow patient.
Eventually returning to Malibu, Marlowe finds the Wade residence being stripped by a real estate company and Eileen mysteriously absent. He heads back to Mexico, where he tries to bribe a local official with the same $5,000 bill. The official reveals a shocking truth: they were paid to stage Terry Lennox's suicide. Marlowe discovers Terry living in a luxurious Mexican villa, where Terry confesses to Sylvia's murder, claiming that he acted in self-defense during a violent confrontation after Roger revealed their affair.
With a sense of grim resolution, Marlowe fatally shoots Terry. As he walks away, he passes Eileen, who is en route to meet Terry. She turns to gaze at him, and in a moment of reflection, Marlowe takes out the harmonica and plays it while leisurely strolling down the road, a haunting melody echoing the tragic tale that has unfolded.
