The Secret of the Old Mill

The Secret of the Old Mill is a juvenile mystery novel by Franklin W. Dixon, first published in 1927; the original text was written by Leslie McFarlane under that house name. It is the third Hardy Boys investigation.

Frank and Joe Hardy expect their detective father at Bayport station but instead encounter a stranger who exchanges a counterfeit five-dollar bill. Fenton Hardy's own case and the boys' first clues soon reveal counterfeit bills across Bayport.

The brothers connect suspicious strangers, repeated warnings, and an apparently deserted mill whose claimed breakfast-food experiments do not match its guarded activity. Surveillance, capture, and concealed access turn the old mill into a criminal machine.

The mystery combines school life and local geography with pursuit by motorcycle and motorboat. Frank and Joe advance by comparing evidence and taking risks together, making brotherhood a practical detective method.

"The afternoon express from the north steamed into the Bayport station to the usual accompanying uproar of clanging bells from the lunch room, shouting redcaps, and a bellowing train announcer. Among the jostling, hurrying crowd on the platform were two pleasant-featured youths who scanned the passing coaches expectantly."