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Playboy private investigator Duke Wallace travels the world solving cases and saving vast sums of money for an insurance company run by Richard M. Rogers. One day, Rogers sends him to Los Angeles to investigate the disappearance six days earlier of one of their agents, William Collins. First, Duke visits Bill's wife Lois at her home. Lois admits that though Bill had many affairs, always with wealthy blonde women, she loved him so much that she forgave him. Assuming that a man with such an attractive, loving wife would have no need to wander, Duke deduces Bill must have been after money. With this in mind, he researches Bill's last few insurance investigations, and comes across a jewel robbery case he considers suspicious, involving the theft of a diamond necklace owned by Martha Carr. At the insurance company's local office, run by Kelly and his secretary Betty, Duke reenacts the night during which Henry and Martha Carr were robbed of the necklace, for which the insurance company subsequently paid. Because the notes state that Martha used the word "automatic" instead of "gun," Duke realizes that the story is false, and visits Martha to question her. Blonde and seductive, Martha deflects Duke's inquiries, and he is soon called away to a cabin in the mountains, where Bill's dead body has been found. There, Kelly is awed by Duke's ability to glean clues from the smallest of details: the presence of an onion in the kitchen and matches in the ashtray convince Duke that a woman was present, and the treacherous mountain roads persuade him that another man was there to drive her from the scene of the crime. Duke returns to Martha and presses her to admit that she dated Bill years earlier. Later, Duke reads through Bill's previous case, in which lovely blonde Helen Fortner inherited a fortune after her drunken husband was killed in a car accident. Researching the night of the murder, Duke learns that it marked the first time Helen refused to drive with Fortner, even though he had been drunk many times before. At Helen's house, she welcomes Duke into her bedroom and allows him to question her butler, Louis, who concurs that she was home the night of Bill's murder. Duke returns to Martha's and states his belief that she arranged the jewel robbery with Bill in order to collect the insurance reward, then killed him after he demanded money to keep quiet. Martha denies the charge, then kisses Duke. He goes back to Helen's and secretly checks Louis' books, sneaking out while Helen dons a negligee. The next day at Kelly's office, they learn that Bill had $10,000 in a secret bank account. Duke questions Lois, and after discerning that she knew nothing about the money, kisses her. Duke then discovers that Henry pretended to leave town the night of Bill's murder but actually did not fly out of L.A. until the following morning. The mystery of where Henry was that night is soon solved when Duke goes to Henry's office and punches him, causing his pretty secretary, obviously his mistress, to embrace Henry in tears. At Lois' apartment, Duke finds Charlie Walsh, Bill's best friend who is clearly interested in Lois. When Charlie responds to Duke's presence with suspicion, Duke is inspired to investigate him, and discovers that Charlie is a bartender who knows Helen and Martha, but when Duke questions Charlie directly, Charlie punches him. Duke returns to Helen's, convinced by the shoddy case notes that Bill was involved in her husband's murder, and there finds the same matchbook as he saw in the cabin. Finally, Helen admits that she had an affair with Bill, who invited her to the cabin, but Charlie, with whom she was also involved, forbade her from going. Duke returns to Charlie's apartment and the two fight brutally until Duke subdues Charlie and has him arrested. The next morning, Betty reads the conclusion of the case: Plotting to get rid of Henry, Helen and Charlie drugged him, then Helen publicly refused to drive with him. Charlie followed him home and pushed his car off a cliff. Then, when Bill discovered the crime, Charlie killed him as well. As Kelly and Betty congratulate Duke, a beautiful woman enters to seek his help on a new case.