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.