One day, Lark and her friend Shelley, visit their neighbour, Ms Trowper, to fetch their ball from her garden. Ms Trowper is not her usual cheery self. She has a pool of water in her attic. She is convinced it is some kind of magic portal. She wants Lark and Shelley to see what's in it. The girls decide Ms Trowper is bonkers and make a hasty retreat.

Later that night, Lark's mum insists she saw Lark's brother, Fin, and his 'sensible' friend, Mat, disappear through a hole in the garden. It makes no sense. Fin and Mat are on a camping trip and are not due home for days.

Troubled by this strange turn of events, the two friends return to Ms Trowper's house. They revisit the hole in Ms Trowper's attic and get sucked through the pool of water into another dimension: Craggthorn, where they are given a very important assignment. If they fail, a powerful spell will bind them to Craggthorn forever.