Best friends Demelza and Nessa are always up for adventure. After all, what else is there to do on the sleepy island of Penfurzy? The two girls have discovered a magic world of myth and legend in a submerged castle just off the island, and now, accompanied by their pet goose Captain Honkers, are on a quest to help a headless knight (or rather, a knight who is JUST a head) find treasure.

They set off on their bikes and explore both the everyday and legendary worlds, and encounter everything from a sullen bully named Connan to a ghost train to an ancient tomb beneath a shuttered amusement park. Along the way, the two girls will become braver and even closer friends.