When Private detective Lew Harper arrives in New Orleans, Louisiana, to meet a new client, he is surprised that the telegram requesting his services, written by Mrs. James Deveraux, was sent by his former lover, Iris. Although Iris and Harper spent a week together six years earlier, Iris explains that she has been with James seventeen years, and that she sent for Harper because she is in trouble. Giving him the name of a motel where she’s made reservations for him, Iris tells Harper to meet her the following day and quickly leaves. Later, at the motel, Harper receives an unexpected visit from a voluptuous teenager and refuses her seduction, but he is detained by Lt. Franks after he sends her away. At the police station, Harper explains that he is innocent, but when police chief Broussard takes a strong interest in Iris’s case, he is unwilling to discuss his client. When Harper is released from custody, he proceeds to Iris’s colonial plantation, Beau Rivage. Iris asks Harper to investigate a letter written to her husband that she intercepted because it exposes her as an adultress. She explains that her husband is an alcoholic playwright and their wealth comes from his mother, Olivia, who has a malicious and authoritative influence in the household. Iris suspects that Pat Reavis, her recently fired chauffeur, wrote the letter and asks Harper to make him stop. On his way out, Iris introduces Harper to her daughter, Schuyler, the girl who attempted to seduce him, and Harper tells her he works for the pest control. Investigating Reavis’ former quarters, Harper finds the typewriter that produced the letter, partially burned, in an incinerator and learns from Schuyler that Reavis showed her a good time. When Schuyler introduces Harper to Olivia in her birdhouse, Olivia resolutely tells him to inform Kilbourne her answer is “no.” Despite Harper’s claim that he does not know Kilbourne, Olivia accuses him of being a “big oil company” man and complains that his business has ruined her town. As he leaves Beau Rivage, Harper spots Schuyler hiding behind a tree and an empty brown car. Back at the motel, two men accost Harper and escort him through a bayou to meet oil tycoon Jay Hue Kilbourne. He tells Harper that he aims to take control of the largest section of oil-rich tidelands, but they are owned by Olivia, who wants to turn them into a bird sanctuary. Kilbourne plans to blackmail the family with salacious photographs of James with male lovers and asks Harper to exploit Iris’s inclination for infidelity. Later, Harper inquires about Reavis at a bar and learns that his girlfriend’s name is Gretchen. Lt. Franks arrives at the bar and brings Harper to a crime scene where Broussard shows him the murdered corpse of Olivia and questions him about Reavis. When Harper consoles Iris, she tells him that she wants things to be the way they were six years ago and kisses him. Back at the bar, Harper picks up Gretchen, who is a prostitute, and coaxes her into telling him where Reavis stays in New Orleans. Searching Reavis’s room, Harper picks up a phone call from room service and learns that Reavis’s sister, Elaine, had called, then finds her business card next to a photograph of Schuyler. Before he leaves, Harper is accosted by two men and thrown into the backseat of a car with an alluring woman, who is informed that neither Reavis nor the account book were uncovered. The woman seductively teases Harper then throws him out of the car as they drive away. The next day, Harper follows Elaine to her apartment and notices the brown car that was parked in front of Beau Rivage. Inside, Harper finds Reavis, holds him up with Reavis’s gun, and discovers an envelope with $10,000. Harper accuses Reavis of killing Olivia for the money and calls Lt. Franks to tell him they’re coming to the station, but Reavis claims he is innocent and warns Harper that he is into something much deeper than he can handle. Driving to the station, Reavis tells Harper that he was fired because he was having sex with Schuyler and knows nothing about the letter. The car is run into a swamp by a truck and Reavis is shot dead. When Broussard claims he never got the message that Harper was bringing Reavis in, Harper suspects that Franks is involved, but Broussard insists that the case is closed. Harper informs Gretchen about Reavis and she blames him for the murder, but when he tells her that he wants to discover the truth and speculates that she has the account book, she gives it to him. Looking through the book, Harper learns that Kilbourne illegally paid off lawmakers for his oil leases. Back at Beau Rivage, James fires Harper, but Harper assures Iris that he won’t quit and tells her Schuyler wrote the letter. That night, Harper accosts Franks inside his home, threatens him with Russian roulette and learns that Kilbourne paid him $5,000 to kill Reavis. On Kilbourne’s yacht, Harper is introduced to his wife, Mavis, the woman who inquired about the account book. Harper reveals his knowledge of Kilbourne’s payments to to Reavis and Franks, but Kilbourne insists that Reavis was not paid to kill Olivia. He claims the $10,000 was blackmail for Reavis because he threatened to implicate Kilbourne in the crime. When Kilbourne warns Harper that his life is in danger, Harper informs him that he has the account book. On his way out, Mavis tells Harper that she fears that Kilbourne will discover that she gave Reavis the account book and begs him to help her. Harper calls Iris to report that Olivia was not murdered by Reavis, but Iris drunkenly demands that he prove Reavis guilty and tells him to get out of her life. Meanwhile, Schuyler is listening to the conversation on another line. After he hangs up, Harper is knocked out and awakens to the jet of a firehose in an abandoned hydrotherapy room with Mavis. Kilbourne leaves them locked inside for the night when Harper refuses to give him the account book, but as they try to escape by flooding the room, Harper is unable to release the skylight and they are trapped as the water rises. When Harper and Mavis become submerged, Kilbourne’s crony opens the door, releasing torrents of water, and they escape to find that Kilbourne has been paralyzed. Harper gives Kilbourne’s gun to Mavis, then attempts to call Broussard; but before he can contact the law, Mavis shoots Kilbourne dead, claiming that he would have gotten away with his crimes if he survived. At Beau Rivage, Harper discovers that Iris has died from an overdose. Emotionally overwrought, Broussard tells Harper about his seventeen year relationship with Iris. Outside, Harper accuses Schuyler of killing Olivia and tormenting Iris so that she could be the center of her father’s attention. Broussard overhears the conversation and slaps Schuyler, then embraces her and apologizes. As Schuyler screams that she hates him and runs away, Broussard attacks Harper and threatens to kill him, but confesses that he is Schuyler’s father. Before returning to Los Angeles, Harper gives Gretchen $9,200 dollars, telling her it was Reavis’s dying wish. When Gretchen offers him the account book, he laughs and instructs her to send it to the biggest newspaper in New Orleans.