In the center of Luster towers an enormous tree that is the Heart of the World. Now the tree is wounded, pierced by a tunnel that connects it to Earth. Through that tunnel strides Beloved, the ancient enemy of the unicorns, followed by an army of Hunters eager to destroy every last unicorn.

At the center of this great struggle is the human girl, Cara Diana Hunter, who sets out with her friend, the gryphon Medafil, to ask a dragon to aid the unicorns. As she travels Cara meets not only the dragon, but a mysterious stranger who may hold the key to Luster's secrets.

As the threads of destiny weave about her, Cara will discover at last the truth about the unicorns--a truth that brings hard choices for everyone she loves as this magnificent, mutli-stranded saga thunders to its astonishing conclusion.