The Secret of the Old Mill

Set in the small town of Bayport, this mystery follows brothers Joe and Frank Hardy as they investigate counterfeit money that has surfaced after a friend receives fake bills. Despite receiving anonymous threats to abandon their inquiry, the determined siblings delve deeper, leading them to an abandoned mill where they uncover a secret passage and the truth behind the counterfeit scheme. This thrilling adventure showcases the brothers' resourcefulness and courage, making it a standout in the beloved series.