Constance York dies in a locked apartment, entirely on her own. It's murder. Months after the case is given up as unsolvable, an anonymous letter comes to Scotland Yard. As an entire family is dragged into suspicion and new crimes come to light, Chief Inspector Reginald Morgan must dig through the evidence and find out exactly what happened. His chances look bleak. His suspects seem never to have met the victim. The murderer could have been anywhere at the time of the incident. Catching the perpetrator of the perfect crime is no easy task, but a plan is beginning to form in his mind.