Author: Franklin Dixon
Original language: English
Counterfeit money is in circulation, and the limit is reached when Mrs. Hardy takes some from a stranger. Then the Hardy boys and their high school chums are warned to stay away from the old mill. Their curiosity plunges them into peril of the most unexpected kind.
Determined to learn the secret of the old mill, Frank and Joe employ a clever use to gain entrance and become trapped. There they unravel two mysteries, one involving a counterfeiting case and the other, a national security case their father is working on.