Los Angeles newspaper reporter Tim Rourke believes that police detective Pete Rafferty is not doing enough to find the murderer of three gamblers. Although his boss, Walter Bronson, orders him to stop criticizing Rafferty, Tim sneaks in a story stating that casino owner Hank Brenner is fronting for a gambling syndicate. Claiming to have affidavits confirming his statements, Tim also reveals that each of the three dead men had won heavily before he was killed and each was seen in the company of a beautiful blonde. Tim then wires his friend, peanut-loving private detective Michael Shayne in San Francisco, asking for his help in solving the murders. In the meantime, Bronson's wife Muriel, a blonde who is attracted to Tim, comes to his apartment after he has been beaten and his apartment searched. Although Muriel suspects her jealous husband is responsible, Tim says that Brenner's men beat him. He adds that someone also searched his apartment for the affidavits. Later, Tim discovers that Muriel is carrying a .32 automatic. Putting together the fact that blonde Muriel has been gambling heavily, with the fact that the gun that killed the three gamblers was a .32, Tim questions her about her possible involvement. In San Francisco, Mike receives Tim's telegram and is about to turn down his request for help, when he reads in the paper that Tim has been seriously wounded, and together with his secretary, Phyllis Hamilton, hurries to the hospital. After checking on Tim, Mike sends Phyllis to talk to Minerva Higgins, Bronson's secretary, while he proceeds to Tim's apartment. When Mike arrives at Tim's, two of Brenner's men beat Mike up when he insists that he does not have the affidavits. Later, Mike tells Phyllis that police chief Will Gentry told him that the fingerprints of two women were found in Tim's apartment. Meanwhile, Phyllis has found a perfumed letter addressed to Tim, containing a phone number and an offer of information. Mike discovers that the number belongs to Madge Rankin, but when Mike knocks on her door, there is no answer. Madge's neighbor, Helen Porter, explains that she has not been home for several days. Mike breaks in to Madge's apartment and discovers that Madge, a blonde, has been killed. Later, the police and Phyllis arrive at the scene. Rafferty takes Mike to police headquarters for questioning, and Phyllis stays behind to talk to Helen. While she is there, a well-dressed man named Dillingham "Dilly" Smith arrives, but says that he had only a slight acquaintance with Madge. At the police station, Rafferty reveals that although all the victims were shot with a .32, none of the bullets came from the same gun. After Mike is released from jail, he and Phyllis proceed to Dilly's. From the doorman, Mike learns that Dilly's wife is a beautiful blonde. Mike tails Dilly and sees him mail a letter to Bronson. Mike then asks Minerva to listen in on Bronson's telephone calls. Bronson has Minerva place a personal ad in the newspaper that says "Yes. Colt." and then receives a call from Dilly, who offers to sell him a gun. When Minerva conveys this information to Mike, he asks Gentry to pick up Dilly and run ballistic tests on the gun. He then summons Rafferty to Bronson's office. Bronson explains that although Tim was shot with a gun registered to Bronson, Muriel had been knocked unconscious before the shooting. Dilly found the gun outside Tim's apartment, took the license number of Bronson's car and blackmailed him. Phyllis then summons everyone to Helen's and reveals that Helen's prints match both the fingerprints in Tim's apartment and those of Dilly's wife. Phyllis also discloses that Helen had recently dyed her hair brown. Mike explains that Helen, who was posing as Dilly's wife, transferred a new barrel into the gun after each murder. To implicate Muriel, Helen slipped the barrel that shot Tim into Bronson's gun and left it outside the apartment. Madge was killed because she was threatening to tell Tim what she knew. Before they return to San Francisco, Mike and Phyllis visit Tim in the hospital and learn that he will recover.