The Secret of the Old Mill is the third volume of the original The Hardy Boys Mystery Stories. This book was written for the Stratemeyer Syndicate by Leslie McFarlane under the pseudonym Franklin W. Dixon in 1927.

After becoming victims of a counterfeiting ring, the boys become suspicious of the activities surrounding a previously disused mill on a deserted road by the river. When they give a boat ride to a stranger with a somewhat suspect story, and the stranger steals the boat and tries to make his way up the same river, they become more interested in the matter and set about to discover what secrets might be hidden at the old mill. Using their inherited sleuthing skills, Frank and Joe succeed in solving the case and bring the perpetrators to justice with the assistance of their famous detective father, the state police, and the Secret Service.

There are plenty of interesting characters to meet along the way along with the usual youthful hijinks. (Summary by James R. Hedrick)