The Secret of the Old Mill

The Hardy boys are determined to learn the secret of an old mill and employ a clever ruse to gain entrance. But soon they are trapped.

In this adventure, there is danger on two levels for Frank and Joe. First, they confront a nasty gang who will stop at nothing to halt their investigations and second, their father Denton Hardy is on a confidential mission that may deal with sabotage.

Will the Hardys unravel the mysteries?