An express train rushing toward the north of England; a group of exceedingly mysterious characters; suspicious happenings in rapid succession; a humorous yokel; a bicycle accident; a lady in distress; and after much mystery a satisfactory conclusion, making this Farjeon story one of the best of its kind. Young Freddy Reeve wants an adventure, but may get more than he bargained for when he encounters the lovely Miss Lydia Leveridge, who provokes more questions than she answers. Freddy finds himself questioning the actions of various fellow passengers, but is quickly thrust into the wake of a criminal plot. He jumps at the chance to save the girl, which brings even more trouble than he imagined. The fast-paced plot leads to a dangerous climax, where several lives are on the line.

The 5.18 Mystery was first published in 1929.