On a magical island on the edge of the sea is the city of Asteri, whose walls, made of trees from the nearby forest called the Barrow, are imbued with the magic of the trees. In Asteri is the boy called Oscar, a shop boy for a powerful magician who rarely gets out of his room. However, when a mysterious illness plagues the island, it falls to Oscar to discover the source of, and the cure to, the illness.