The Aldens notice that someone is keeping a close eye on their new neighbors. Can they figure out why?

Henry, Jessie, Violet, and Benny aren't expecting a mystery when they offer to show their new neighbors around Greenfield. But when they uncover a series of coded messages hidden in historical documents, they realize they might still have a lot to learn about their hometown's past. The situation gets even more suspicious when the Aldens realize someone is trying to stop them from discovering the truth. Will they be able to crack the code and uncover Greenfield's true history before the evidence is lost forever?