Franklin W. Dixon 
The Secret of the Old Mill 

Frank and Joe help prevent Ken Blake from getting killed in a cycling accident. The Hardy Boys learn that their friend Chet Morton was swindles when he made change for what turned out to be a counterfeit twenty dollar bill, something that the police later confirm is becoming more common in their town of Bayport.

Later, Joe is awakened by a clattering sound and sees a mysterious figure bicycling away from the his home. While investigating the disturbance a note is found that reads, “Drop the case or else danger for you and your family.” The Hardy boys are not sure if this threat refers to the counterfeiting case that Frank and Joe are investigating or another case their detective dad, Fenton Hardy, is trying to solve, but they’re intent on finding out who’s threatening them. Can they stay alive and solve both cases before danger finds them?