Aunt Alberta has stolen Stella's home and tried to kill her for it. The novel is about reclaiming what is rightfully yours.
Stella and Sab — a living girl and a long-dead ghost — form a warm and genuine friendship. Walliams presents friendship as transcending even death.
In the Dahl tradition, the villainous aunt meets a satisfying comeuppance. Walliams's moral universe is clear: wickedness is punished.