Stage 9 in the Oxford Reading Tree series marks an exciting step forward in children's reading development. At this level, readers are typically 7–8 years old and are beginning to read more confidently, fluently, and independently. Stage 9 books include longer, more detailed narratives, richer vocabulary, and more complex sentence structures, preparing children for chapter books and wider reading.

In Storm Castle, Biff, Chip, Wilf, and Wilma are transported by the Magic Key into a high-tech virtual reality game world. They arrive in a dark and stormy landscape and soon realise they must complete a quest to escape from the ominous Storm Castle. The children are each given roles and must work together to solve challenges, dodge traps, and outsmart a sinister figure known as the Game Master. The tasks become more dangerous as they get closer to the castle, including puzzles, timed escapes, and team-based obstacles. Using logic, bravery, and cooperation, they finally make it to the top of the castle and complete the game — just in time for the Magic Key to glow and take them home.