Join America's favorite teen sleuths in this exciting tale filled with mystery, suspense, and unexpected twists. Perfect for fans of action-packed, page-turning adventures. Amateur sleuths Frank and Joe Hardy investigate a counterfeit operation while their detective father is connected to a secret case involving national security. An old mill on the outskirts of town appears to hold the key to solving both crimes. Determined to learn the secret of the old mill, Frank and Joe devise a clever scheme to gain entrance, only to find themselves trapped in an underground tunnel. Ranking on the All-Time Bestselling Children’s Books list, the third installment of the Hardy Boy series will have readers on the edge of their seats.   Solve mysteries with the Hardy Boys: Teen detective brothers Frank and Joe team up with their detective father to crack another case! Follow along as they unravel a counterfeit operation and uncover a secret connected to national security. - The Mysterious Mill: An abandoned mill on the outskirts of town holds the key to both cases. Can the Hardy Boys unlock its secrets before it's too late? - Action-packed adventure: Determined to solve the mystery of the old mill, their investigation takes an unexpected turn when they find themselves trapped in an underground tunnel. Frank and Joe must use their wits and courage to escape. Can they bust the bad guys and solve the case? - Classic Sleuthing: Experience the thrill of the Hardy Boys' signature detective work, filled with clever clues, daring chases, and nail-biting suspense. - Classic children's book: This third installment in the Hardy Boys series remains a timeless favorite among young readers. Perfect for fans of mystery, adventure, and clever sleuthing. - Join the ranks of loyal fans: With its compelling plot and relatable characters, it's no wonder this book has earned its spot on the All-Time Bestselling Children's Books list. Don't miss out on this thrilling read! - 160 pages, 5 x 8 Inches. Ranking on the All-Time Bestselling Children’s Books list, the third installment of the Hardy Boys series will have readers on the edge of their seats. Amateur sleuths Frank and Joe Hardy investigate a counterfeit operation while their detective father is connected to a secret case involving national security. An old mill on the outskirts of town appears to hold the key to solving both crimes. Determined to learn the secret of the old mill, Frank and Joe devise a clever scheme to gain entrance, only to find themselves trapped in an underground tunnel. Brimming with fast-paced action, the Hardy Boys mystery series continues to delight countless readers since first being published in 1927. Franklin W. Dixon is the pen name used by a variety of different authors who were part of a team that wrote The Hardy Boys novels for the Stratemeyer Syndicate.