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.

Franklin W Dixon : was the pseudonym devised by Edward Stratemeyer for the author of a series of mystery books he was developing which became the Hardy Boys series. The first book, The Tower Treasure, originally published in 1927, was written by Leslie MacFarlane who went on to write 19 more, including #2 through #16. In all, there are 58 titles in the original Hardy Boys Mysteries series published between 1927 and 1979 written by 17 different men and women. Many of the books were later revised, adding another four "Franklin W. Dixons" to the total.