"Drop case or else danger for you and your family."
The warning addressed to Fenton Hardy not only alarms
the famous private 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 can-
not 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 Elek-
ton employees who are hostile to Frank and Joe and re-
fuse to allow the young detectives to question the com-
pany's fourteen-year-old messenger boy who unknowingly
may 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.

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