A resourceful ten-year-old who has moved from a smoky city to the countryside becomes a keen Cub Scout and applies his love of detection to a series of local mysteries. Through outings, night watches, and signalling, he uncovers suspicious activity around an old abbey and nearby mill pond, follows dark passages, and helps expose a clandestine group stockpiling arms. Using observation, signalling skills, and Scoutcraft, he aids his fellows in capturing the culprits and restoring safety, earning the troop's praise.