Dr Riddle is trying to remember and putting on paper in detail the events that have happened on the same day. He was coming back home from Connecticut to NYC when his car had a mechanical problem in the middle of the forest. He searched for help and happily found the house of Dr. Mac Comerou, a professor who was the author of the definitive book about the psychology of murderers. After a while, Riddle left the house and soon stopped when he saw a young girl walking on the road.
The young girl, Elinor Darries, tells him that her fiancÚ, Inis Saint-Erme, has just been kidnapped by a kind of bum who wore a blue hat and who stole the couple's car. Back at Mac Comerou's house, Dr Riddle is told by the police that the car of the kidnapper hit a man on the road and that several witnesses had seen the bum behind the steering wheel, laughing, while an unconscious Saint-Erme was seen at the car's window.
Soon, Saint-Erme's body, with the right hand missing, is found in the swamps and Mac Comerou coldly murdered. It appears that the murderer wants to kill anyone trying to discover his real identity. Meanwhile, all the clues found by the police lead to Dr Riddle who wants to pass the night analyzing the events of the day. He's also told that he is the murderer's next victim.
This report prepared by Daniel Staebler