The Secret of the Old Mill is the third book in the Hardy Boys series by Franklin W. Dixon. In this thrilling mystery, Frank and Joe Hardy tackle a case involving counterfeiting and danger while uncovering the secrets of an old mill in their hometown of Bayport.

Frank and Joe Hardy, along with their friend Chet Morton, get involved in a new mystery after their father, the famous detective Fenton Hardy, reveals that counterfeit money is circulating in Bayport. At the same time, a series of odd events point to the old mill outside town, which seems to be tied to the counterfeit operation.

As the boys investigate, they face numerous challenges, including suspicious characters, attempts to intimidate them, and the mill's dangerous, labyrinthine layout. They discover that the counterfeiters are using the old mill as their base of operations. After a tense confrontation and some clever detective work, the Hardy brothers manage to foil the gang's plans and bring them to justice.