Tuesday and Zed Furst are two seemingly normal children, but their parents are anything but. Their father is secretive about his work, their mother is always accompanied by a guard dog, and they are forbidden to talk about their family tree.
When their parents go away on a "business trip," Zed and Tuesday decide to investigate their parents' secrets. However, they are attacked by thugs with shape-shifting weapons and are forced to flee their home. They end up in the modern-meets-medieval world of Falinnheim, where everyone believes their father is a fugitive.
The siblings soon realize that they need help if they want to find their parents and get back home. They team up with a smuggler with a broken compass, their unusually talented dog, some extremely organized bandits, and a selection of suspiciously misquoted nursery rhymes.
Along the way, Zed and Tuesday learn more about their parents' secrets and their own past. They also discover that what is normal is not always what it seems.