The Secret of the Old Mill - Franklin W. Dixon

The Hardy boys are determined to learn the secret of an old mill and employ a clever ruse to gain entrance.\n But soon they are trapped.\n \n In this adventure,\n there is danger on two levels for Frank and Joe.\n First,\n they confront a nasty gang who will stop at nothing to halt their investigations and second,\n their father Denton Hardy is on a confidential mission that may deal with sabotage.\n \n Will the Hardys unravel the mysteries?\n