When Cally breaks her leg, she starts to imagine melodramatically that she is going to die of the complications or be an invalid for life. Instead, the experience (with some help from a brave and energetic boy and a strong teacher) helps Cally to realize that she can do all sorts of things. It teaches her to stand up for what she wants, rather than doing what her parents or other people want her to do. And it teaches her to be brave in the face of adversity.
Mills write lovely concise books for tweens (probably why I've reviewed so many of them over the past months). This one touches on all sorts of subjects (self-expression, responsibility, believing in yourself) and it avoids getting too preachy about it. Cally is her own teacher, relying on adults from time to time for advice, but generally understanding that only she can change herself. That is a wonderful lesson to impart to children (whether they read this book on their own or have an adult to read it with).