An unknown work from the famous Limbourg Brothers dating back to the Middle Ages turns up in the city archives of Nijmegen. This 'Treasure of Nijmegen' is given a precious place in the permanent exhibit of Museum the Valkhof. But how did that book end up in the archives? Who used to own the book? And who is now claiming to be the owner? The Treasure is moving a lot of people, and not all of them have good intentions. Kim and Eddy, two bright seventh graders, get involved in the strife over the book, too, whilst they're being prepped by aunt Kate for the major walking event the Four Day Marches in Nijmegen.