"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 Milhouse is occupied by two Elekton employees who are hostile to Frank and Joe and refused 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. 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. - Flyleaf.

The Hardy boys determined to learn the secret to the old mill employ a clever ruse to gain entrance only to find themselves trapped in a subterranean passageway.