When Christopher Columbus-wannabe Rupert Lilywhite decides to set sail and discover a new land - it's all the rage in the fifteenth century - Walter Bailey and his twelve-year-old nephew Hugo are employed as the ship's mapmakers. And when the sailors do eventually spot a mystery island, it is Walter and Hugo who are dispatched to investigate (nobody else can be bothered).

Arriving on a beach of purple sand, the intrepid pair believe that they have found paradise . . . until Walter is picked off by a giant flying rat. But with the help of some weird and wonderful talking creatures, including Pigasus the flutterhog, Delphina the water-breathing merphin and Savage, the tiny but fearless mouse, Hugo is given a chance to save his uncle and put his mapmaking training to the test.