Wealthy murder mystery writer Paula Bradford (Jean Arthur) returns from her worldwide travels to see her former husband, surgeon Dr. Lawrence "Brad" Bradford (William Powell). He had divorced her because she was always involving him in real-life murder cases, but she wants him back. When a jockey riding the favorite dies while leading a race, she is convinced it was murder. She and Mike North (Frank M. Thomas), the horse's trainer, persuade the reluctant doctor to investigate. Brad is puzzled when he finds traces of gelatin on the corpse.
Then he receives an envelope addressed to North. North telephones a little later to say he will come pick it up. Curious, Brad opens the envelope to find a great deal of money. Someone claiming to be North calls shortly afterward and instructs Brad to meet him somewhere else, but without the money. Suspicious, Brad takes a taxi, but gets out a short distance away and returns to his suite. As he anticipated, a burglar is inside looking for the letter. When the two men start fighting, Paula tries to help, but ends up knocking Brad out instead, allowing the intruder to get away.
Soon after, the doorbell rings. When Brad opens the door, North's body falls in. Police Inspector Corrigan (James Gleason) names Brad as the main suspect in the second death. Now Brad has to solve both murders to clear himself.
All sorts of suspects present themselves: Nick Martel (Robert Armstrong), a bookie who owed North $100,000 for a winning bet; Leroy Hutchins (Ralph Morgan), the owner of the winning horse; John Summers (Grant Mitchell), the favorite's owner; Mrs. Summers, who is seeing Martel behind her husband's back; and Summer's lawyer, Henry Strand (an uncredited Frank Reicher). Even Brad's receptionist, Miss Prentiss (Lila Lee), had been spotted with Martel. Brad discovers that Martel was instructed to send the money to an address he was given, but decided to confront North in person instead over what he considered to be deceit; though North claimed he did not place the bet, Martel did not believe him and gave him the money. Going to the address provided, Brad finds Paula already there. They then discover the body of Lou Pender (Paul Fix), the would-be burglar, who had masqueraded as North. As Brad is examining the corpse, an unseen person shoots him from the window. Fortunately, he is not seriously wounded.
Brad eventually figures out how the murders were committed. The killer put a deadly black widow spider inside a gelatin capsule, secretly attached it to the victim's body, and waited for the gelatin to melt from body heat, releasing the spider. Since the police have the $100,000, Brad assumes the killer will try the same trick again. The same two horses are scheduled to race against each other the next day, so Brad has film cameras set up around the racetrack. The murderer does strike again, but fortunately, Brad had taken the precaution of inoculating the jockey beforehand.
Brad invites all of the suspects to his suite, hoping the culprit will stay away. When that fails, he screens the film footage he had taken. It shows Mr. Summers slipping a capsule down the jockey's back. Pulling out a gun, Summers admits his motive was revenge against his unfaithful wife and Martel. Brad subdues him before he can shoot the lovers. During the struggle, however, Paula once again knocks out her ex-husband. Despite this, Brad marries her again.William Powell as Dr. Lawrence "Brad" Bradford
Jean Arthur as Paula Bradford
James Gleason as Inspector Corrigan
Eric Blore as Stokes, Bradford's butler
Robert Armstrong as Nick Martel
Lila Lee as Miss Prentiss
Grant Mitchell as John Summers
Erin O'Brien-Moore as Mrs. Summers
Ralph Morgan as Leroy Hutchins
Lucile Gleason as Mrs. Hutchins
Frank M. Thomas as Mike North
The film was very popular and earned a profit of $350,000.