While riding through the forests of Angleland, cousins Per and Radius realize they are lost and are desperate for a map to guide them home. Soon they come across a mysterious house in the hills. Inside they find a map to a treasure belonging to Xaxon Yellowbearyd, the fiercest Viking warrior of his time. Per and Radius must decode the strange numbered grid on the map, while trying to steer clear of the pack of bungling bandits who are on their tail. Will they find the treasure in time?
Cousins Per and Radius hunt for treasure by unraveling a Viking mapmaker's clues: a series of coordinates (x, y) that pinpoint locations on the Viking's map grid. Highway robbers, channeling the Keystone Kops, chase the cousins around, but this subplot comes off as rather superfluous. Acrylic paintings with a medieval setting illustrate this introduction to coordinate (a.k.a. Cartesian) planes.