The Ex-Mrs. Bradford

The killer placed black widow spiders inside gelatin capsules, then attached the capsules to the victims' bodies. The heat from the body would melt the gelatin, releasing the spider to bite and kill the victim. This explains the gelatin traces Dr. Bradford found on the jockey's body and the mysterious deaths that appeared natural at first.

Earlier, Brad notices gelatin on the jockey's body, which initially puzzles him before he connects it to the spider method.

When North's body fell into Brad's apartment, Inspector Corrigan immediately suspected him because North had been murdered in Brad's building, and Brad had recently received a mysterious envelope of money connected to the racing world. The circumstantial evidence made Brad the prime suspect, forcing him to investigate the real killer to clear his name.

Brad received an envelope containing a large sum of money addressed to Mike North. Since the calls about the envelope seemed suspicious, Brad opened it, found the money, and replaced it with torn magazine pages to throw off whoever was looking for it. A thief later broke in searching for this envelope, confirming it was connected to the murder plot.

The mysterious calls directing Brad to meet 'North' without the money hint that something valuable was being concealed.

Mr. Summers killed the jockey, Lou Pender (posing as North), and attempted to kill again because he sought revenge against his unfaithful wife and her lover Nick Martel. He blamed them for his humiliation and wanted to punish both by framing the racing world for murder while getting away with killing them through the spider method.

Mrs. Summers is shown secretly involved with Nick Martel, who owes money to North, establishing the triangle of betrayal.

Brad set up film cameras around the racetrack knowing the killer would attempt another attack during the next race with the same horses. He also inoculated the jockeys against spider bites as a precaution. When Mr. Summers tried to slip a capsule down the jockey's back during the race, the cameras captured the entire act, providing the evidence needed to arrest him.

Brad explicitly states he expects the killer to strike again at the race, showing his confidence in anticipating the criminal's behavior.

Brad divorced Paula because her obsession with solving real murder cases was too much for him to handle. He was a dedicated surgeon who wanted a more stable life, while Paula couldn't resist getting involved in actual criminal investigations, which created an irreconcilable rift in their marriage despite their continued affection for each other.

Lou Pender was a man who masqueraded as Mike North, likely as part of the murder scheme. Brad and Paula discover his body hidden in a Murphy bed after he killed a spider (which was part of the murder weapon that backfired). He was killed using the same spider method, making him another victim in the conspiracy.

Brad proved his innocence by solving the actual murder case rather than simply claiming he was framed. He investigated the suspects, discovered the spider murder method, set up cameras at the racetrack, and captured Mr. Summers in the act of attempting to kill again. By identifying and exposing the real killer, Brad cleared his own name.

During the chaos of Brad confronting a burglar in their apartment, Paula tried to help by grabbing a skull and intervened in the struggle. In the confusion, she accidentally knocked Brad unconscious, which allowed the intruder to escape. This happens again at the end when Paula accidentally knocks Brad out with a vase while helping subdue Mr. Summers. These comedic mishaps highlight the screwball comedy tone of the film.

Paula's tendency to insert herself into danger despite good intentions is established early when she insists on investigating the jockey's death against Brad's warnings.

Nick Martel was a bookie who was indebted to Mike North for a large sum of money. This financial connection linked him to the web of suspects surrounding the racing world murder plot. While not the killer, his debt and secret relationship with Mrs. Summers made him a suspect in the investigation.

Brad inoculated the jockeys with an anti-venom before the crucial race. This medical precaution was part of his plan to trap the killer, if the spider was released, the inoculation would prevent the jockey from dying, allowing the killer's attempt to be captured on film without resulting in another death.