Sir Cumference and the Isle of Immeter

When young Per visits her uncle Sir Cumference, aunt Lady Di of Ameter, and her cousin Radius, they teach her how to play Inners and Edges. After Per finds a clue linking the game to the mysterious castle of the Countess Areana, she and Radius sail to the island of Immeter. There, they have to decipher cryptic clues while avoiding a sea serpent. To unlock the island's secret, Per has to figure out how to find the perimeter and area of a circle. Only then can she become Per of Immeter.