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Dennis O'Brien operates a small boat rental business on the San Francisco waterfront, and moonlights as an amateur detective. One day, Dennis attends an auction, where he is approached by an attractive woman, Claire Underwood, who offers to pay him if he will bid on a black leather suitcase that she says belongs to her. Stiff bidding for the locked suitcase drives the price up to $1,000, but Dennis succeeds in buying it with Claire's money. While he is collecting the suitcase, a police officer appears at the door, and Claire vanishes. Dennis takes the suitcase home and opens it, and is surprised to see that it contains only a saxophone. He is then visited by Larry Dunlap, the man who bid against him at the auction, who demands the saxophone. Dennis agrees to sell the instrument for $100, but Dunlap knocks him out and steals it. When Dennis comes to, homicide inspector Lt. Bruger is standing over him. Bruger says that Dunlap robbed the auctioneer, Cole, and that a musician named Charlie Bebe--the owner of the saxophone--was shot to death. After Bruger leaves, Dennis seeks advice from his former roommate, the alcoholic Prof. Shicker, who agrees to research the matter. He then goes to see Cole, and intimidates him into admitting that he had agreed to sell the saxophone to his friend, musician Bud Becker. Dennis drops by the Down Beat Club, where Bud plays, and when he encounters Claire backstage, accuses her of killing Charlie for his saxophone. Claire replies that Charlie was her boyfriend, and asks Dennis to meet her later at her hotel. Dennis then meets with Shicker, who informs him that Claire has served time for shoplifting and Dunlap is a fence. Dennis goes to Claire's hotel room, where he is held at gunpoint by Dunlap, and they are soon joined by Bud, who says Claire asked him to meet her there. Dunlap leaves, and Bud suddenly collapses and dies from poison. Later, Shicker calls Dennis from the police station to report that Dunlap and Charlie had been forming a partnership behind Claire's back, and that someone has attempted to frame Dennis by booking a flight to Mexico in his name. Dennis goes to the airport, where he finds Claire. Dunlap comes to take the saxophone, but Dennis subdues him, and Bruger and his men arrest Claire. After Bruger says that he has been tailing Claire since she opened a safety deposit box at the bank, Dennis takes the saxophone apart to reveal a fortune in jewels hidden inside. Dennis concludes that Claire and Charlie had worked as a team of jewel thieves until she killed him, and sadly observes that Bud was the only one who wanted the saxophone because it made beautiful music.
One day, Dennis gets a new case when he is visited at his shop by disreputable private detective Edgar Spadely, who offers him money to chaperone his client, the wealthy Vicki Jason, to a yacht party that evening. Dennis accepts the assignment, then consults Shicker, who says that Spadely is a confidence man and racketeer. When Dennis accompanies Vicki to her yacht, he is surprised to see that there is no party. Vicki flirts with him, and as they are kissing, Spadely enters, accompanied by Vicki's estranged husband Arnold, and takes a photograph. Arnold then threatens to name Dennis as co-respondent in the upcoming divorce proceedings. The following morning, Dennis returns to the Jasons' yacht and finds Arnold dead. He then meets Arnold's niece, Sheila, who recognizes Dennis from the photograph and accuses him of murder. Dennis suggests that Vicki killed her husband for his money, but Sheila replies that her uncle's will left everything to her. Back at his shop, Dennis is visited by Bruger, who suspects him of the killing. After Bruger leaves, Dennis calls on Sheila, who says she now believes that Vicki is the murderer, because the final entry in Arnold's diary revealed his fear that his wife would kill him if he went through with the divorce. After calling Bruger, Dennis goes to see Spadely and accuses him of being in cahoots with Vicki. Dennis tries to inspect his files, but Spadely knocks him out and flees. When Dennis comes to, he receives a phone call from Bruger, who says that Vicki told the homicide detectives that Dennis killed her husband. Bruger then reads an obviously fabricated entry from Arnold's diary that implicates Dennis. Before Bruger can arrest him, Dennis meets with Shicker, who informs him that Vicki and Spadely were high school sweethearts, and that Vicki recently asked to be appointed Sheila's guardian. Shicker adds that Sheila instead chose Spadely for her guardian, and that they are heading to Reno to elope. Dennis goes to Spadely's apartment, and they are joined by Vicki, who admits killing Arnold and shoots Spadely. Bruger comes in and arrests Vicki, and Dennis escorts Sheila home.