The Secret of the Old Mill

“Drop case or else danger for you and your family.” This warning addressed
to Fenton Hardy not only alarms the famous investigator and his detective sons,
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 since it involves national security.
The key to the solution of both cases appears to 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
company’s fourteen-year-old messenger boy who unknowingly may be in
possession of some vital clues.