Fifth graders Lizzie and Zach love playing hockey, being outside, and climbing the rocks at Raven Park. When it's decided that they'll spend a whole day at the park, the two friends can barely subdue their excitement. This sunny day will be different from all the others, though, as Lizzie and Zach come face-to-face with the mysterious park guardians-the stone people. The kids can't be sure about the stone people. Are they friends or foes? Do they mean to do harm, or are they there to protect the park and all those who enter? Lizzie and Zach decide to answer these questions by using their imagination, and soon they travel to far-off places and take on big adventures, all in a world of their own creation. When Lizzie begins to feel sick, though, it's hard to trust the stone people. Lizzie's parents come to realize she has juvenile diabetes. Her condition worsens, and even best friend Zach can't imagine this illness away. As Lizzie gets sicker, the stone people grow bigger and bigger and possibly, more sinister-or maybe not. Maybe the stone people aren't sinister at all. Maybe they're preparing for battle as Lizzie's personal guardians against the illness that threatens to steal her away.