When the death of wealthy, eccentric gun collector, Gordon Bates turns out not to be suicide, as first thought, but murder, young Lieut. Jim Stevens is paired with veteran police detective Sgt. Boggs to uncover the killer. Boggs immediately assumes that Lou Ann Winton, the dead man's fiancée, is guilty because her fingerprints were found on the gun. She admits that she struggled with Bates when he wanted her to become his mistress, but denies that she murdered him. As the forensics laboratory uncovers each new piece of evidence, Boggs transfers his suspicions to a different suspect, and Stevens, who was once Lou's lover, must clear each one. The lab identifies the hair under Bates's nails as belonging to Lou's brother Jack. Then the laboratory reveals that the gun with Lou's fingerprints was not the murder weapon. Stevens begs Lou to tell him the whole truth. She admits that she still loves him and agreed to marry Bates only because he was blackmailing her mother. With the help of Bates's valet, Horton, she and Jack were trying to retrieve the letters that were being used in the blackmail. Stevens believes there is still more to the story. When rug dealer Anderzian demands the return of some letters from Bates's safe, Stevens reads them to see if he can discover a motive for murder. Finally, the laboratory informs Stevens that on each letter, a second letter has been written in invisible ink and reveals Anderzian's part in the blackmailing scheme. To cover his part in the crime, Anderzian kills a man who saw him in the house on the evening of the murder. After Stevens and Boggs arrest him for Bates's murder, Horton confesses that he killed Bates in self-defense while he was trying to remove Lou's mother's letters from the safe. Stevens suggests that Horton will be acquitted if he pleads self-defense, and then proposes to Lou, who happily accepts him.