The Secret of the Old Mill by Franklin W. Dixon - is Volume 3 in the original The Hardy Boys Mystery Stories published by Grosset & Dunlap. The book ranks 86th on Publishers Weekly's All-Time Bestselling Children's Book List for the United States, with 1,467,645 copies sold by 2001. This book is one of the "Original 10", some of the best examples of the Hardy Boys, and Stratemeyer Syndicate, writing.
Frank and Joe Hardy and their friend Chet Morton wait at a train station in their hometown of Bayport for Frank and Joe's father. While there, they see a man jump on a departing train. Frank is 18, and Joe is 17. Their father is a famous detective and is returning from a business trip to Detroit. When he doesn't arrive, the brothers assume he has been unexpectedly delayed and plan to meet a later train. In the meantime, the three friends go into town to pick up Chet's new microscope.
Ken Blake, a teenager who works as a bicycle errand boy, is almost hit by a car. Frank, Joe and Chet help him. Joe gives Ken an envelope he dropped, noticing it is labeled confidential and is addressed to Victor Peters. Ken thanks the boys but is vague about where he works. Frank inspects Ken's bike, and Ken tells him it came from Belgium.
When Chet pays for the microscope, the shopkeeper tells the boys that one of Chet's $20 bills is counterfeit. The police have warned local shopkeepers about a counterfeit ring. Frank asks the shopkeeper if he can take the bill to study it and then turn it over to the police. The shopkeeper agrees. The shopkeeper lets Chet take the microscope and owe the balance.
Chet asks to borrow $20 from his father, a real estate agent. He tells the boys about the property outside of town that he sold to Elekton Controls. The company manufactures missile parts, and the land purchased includes an old mill. At Chet's father's suggestion, the company renovated the mill. As the boys are getting out of Chet's car back in downtown Bayport, they see the man from the train station but are unable to catch him. The boys pay the balance due on the microscope and go to the police station.
At the police station, the three friends learn that other townspeople have also received counterfeit bills. The man Chet saw matches the police description of one of the suspected counterfeiters. The boys go back to the train station, pick up Frank and Joe's father and head home. Mr. Hardy doesn't think the counterfeit case is related to his case, but he offers to help his sons track down clues. He also tells them that Elekton is doing experimental work, in addition to manufacturing.
In the middle of the night, Joe hears someone on a bicycle leaving an envelope in the Hardy mailbox. He chases the person without success. Awakened by the noise, the Hardy family, including Frank and Joe's visiting Aunt Gertrude, reads the note and then searches the yard for clues. The note warns of danger for the family if the Hardys don't drop the case - but it doesn't specify which Hardy or what case. Aunt Gertrude finds a bicycle pedal outside.