Set in 1876 in the coal hills of Pennsylvania, the story follows two unwitting orphans, Patrick and Sissy Hughes, who are propelled on a desperate journey after witnessing the murder of their father. They carry with them a wooden box retrieved from a secret compartment beneath their father's bed. His dying words to them: "keep it safe, keep it hidden." Powerful men are looking for what's inside the box and they will do anything they can to get it. One hundred miles to the South, large crowds have gathered at the Centennial Exhibition in Philadelphia. Patrick and Sissy's father had promised to take them there for the Fourth of July celebration. Instead, they are running for their lives. Alone and on the run, they are pursued by their father's murderer, James McKenna, a Pinkerton Security Agent who is working undercover investigating the Molly Maguires for the Philadelphia and Reading Railroad. Patrick and Sissy's only hope is to unlock the mystery of the contents of the box: a diary, a translucent blue stone and a bag of blue ground. That hope lies in Philadelphia with Henry Carvill Lewis, a professor of mineralogy at the Academy of Natural Sciences. As they make their way to Philadelphia, their pursuers grow in numbers and Patrick and Sissy must fight against time and the odds to stay together.