In 'Secret of the Old Mill,' the third installment of the classic Hardy Boys Mystery series, brothers Frank and Joe embark on a thrilling investigation surrounding a deserted mill on a secluded river road. After witnessing the suspicious activities of a stranger and falling victim to a counterfeiting scheme, the boys' curiosity is piqued. They soon discover that the old mill harbors a web of secrets, including a counterfeiting ring. With their sharp intellect and inherited sleuthing abilities, Frank and Joe team up with their renowned detective father, the state police, and the Secret Service to solve the mystery, bring the criminals to justice, and expose the truth behind the old mill's enigmatic facade.