A detective teams with his ex-wife to solve a murder.

When word of jockey Eddie Sands's sudden death while riding in a big race hits the newspapers, amateur sleuth and mystery writer Paula Bradford immediately assumes foul play and shows up at the apartment of her ex-husband, surgeon Dr. Lawrence Bradford. Reluctant to pursue Paula's theory that Eddie was murdered, Bradford, who divorced Paula because of her penchant for involving him in murder cases, agrees to examine Eddie's corpse only after horse trainer Mike North shows him a threatening note found in Eddie's locker. During the examination, Bradford discovers a gelatinous substance on Eddie's arm but nothing else of note. Soon after, however, Bradford receives a money-filled package intended for Mike, a mysterious phone call from someone pretending to be Mike, and a visit from a burglar looking for the package. After fighting the burlgar and getting knocked out by Paula, who has moved back in with her ex-husband, Bradford learns that Paula saw his secretary, Miss Prentiss, with bookmaker and murder suspect Nick Martel just before the corpse of Mike North is found on the Bradford's doorstep. Now the prime suspect of Inspector Corrigan, who informs Bradford that an autopsy on Eddie revealed that he died of a collapsed lung, Bradford visits the three people from whom Mike received phone calls the night he was murdered: Leroy Hutchins, the owner of the winning horse, John Summers, the owner of the stable where the horse was trained, and Summers' lawyer, Henry Strand. With the devoted help of Paula, and visual aid from hired newsreel cameramen, Bradford eventually uncovers an elaborate plot devised by the financially troubled Summers and Mike North impersonator Lou Pender to murder Eddie and another jockey riding against Summers' horse. Bradford reveals that the murder was committed using a black widow spider encased in a gelatinous capsule and placed inside the rider's clothes. Cleared of all suspicion, Bradford, who suffered a gunshot wound from Summers and another knock-out from Paula, proposes to his ex-wife and is re-married in a "previously filmed" marriage ceremony.