A girl from overseas, Joan, comes in the old city of her grandparents in The Netherlands and does spectacular discoveries. The old cathedral and the little river that runs underneath the town in which you can go by boat, are important for her in her adventures. The other story in the book runs about another girl, Janne, who lives in the Middle Ages in the same city with her parents, her father works on the cathedral. Why both the girls seem to meet one another a clever child will find out while reading. There is a very special reason for that.