In this unabridged recording of the third original 1927 edition of the Hardy Boys Detective story The Secret of the Old Mill, brothers Frank and Joe Hardy exchange some one-dollar bills for a five-dollar bill with a stranger at the train station.

Later, when they try to use the bill to buy some pie plates, they are told the five is a counterfeit!

Frank's girlfriend, Callie Shaw's cousin Pollie, accepts a counterfeit fifty from a customer, and the Hardy Boys' mother does the same when selling an old carpet!

During their investigation of the source of the counterfeit bills, the Hardy boys and their high school chums are plunged into peril of the most unexpected kind.

While visiting an old mill, hostile scenes convince the Hardys that something is going on. Putting two and two together, they deduce that the counterfeit money and the mill may be connected. A series of dangerous investigations ensues!