Katie, thirteen, and her twelve-year-old brother Zach have spent their whole lives in the vast and ancient stone fortress called Castle Falcon, somewhere in the present-day American northwest. Their loving but enigmatic father, driven by secret fears, rarely allows them to leave the safety of the Castle and venture outside.

Katie and Zach are constantly testing the boundaries of their enormous gilded cage. When they finally crack the secret of opening the mysterious Castle rooms their father had sealed against their curiosity, they aren't ready for what they find: living hearts in jars, people turned to stone, and a cage containing something terrible and invisible.

And that's before they discover a buried gateway to a dark and demon-filled hell, and the monstrous creature that guards it ...

Katie and Zach thought their summer would be boring. Now, with the help of some very unusual friends, they may have to save the world!