The Hardy Boys Mystery Series revolves around Frank and Joe Hardy, sons of a detective Fenton Hardy, who act as amateur sleuths, solving cases that stumped their adult counterparts. In the Secret of the Old Mill the Hardys learn that their friend Chet Morton has been tricked when asked to make change for what turned out to be a counterfeit twenty dollar bill. That is becoming more common in their town of Bayport, and Frank and Joe begin to investigate this curious case. When the Hardys and some of their friends go nearby an old watermill's to have a picnic, an arrow is shot at them with an attached note to give up their investigation. There is something strange about the old mill and the Hardy brothers decide to find out what.