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.