:"Drop case or else danger for you and your family."
This warning addressed to Denton Hardy not only alarms the famous investigator and his detective sns, but also puzzles them, because Mr. Hardy is working on more than one case. Which one prompted the threat?
When more ominous warnings follow, Frank and Joe suspect there is a link between the counterfeiting case they are investigating and a secret case their father cannot discuss because it involves national security.
The key to the solution of both cases appears o be hidden in the old Turner Mill, constructed in frontier days but now a gatehouse for Elekton Controls Limited engaged in manufacturing top-secret electronic parts for space missiles. But the millhouse is occupied by two Elekton employees who are hostile to Frank and Joe and refuse to allow the young detectives to question the companiesí fourteen-year-old messenger boy who may unknowingly be in possession of some vital clues.
Determined to learn the secret of the old mill, Frank and Joe employ a clever ruse to gain entrance, only to find themselves trapped in a subterranean passageway. How the young detectives extricate themselves from this dangerous situation and unravel both mysteries will keep the reader tense with suspense.
Original title: The Secret of the Old Mill
Genre: Fiction→ Children & Young Adult→ Mystery