Eleven-year-old Amy Witowski has always seen things others can't-shadow entities lurking in reflections, impossible creatures hiding in plain sight, and thin spots where reality itself seems to tear. After her parents' mysterious disappearance, Amy is sent to live with her aunt in Daybridge, a city where the boundaries between worlds run dangerously thin. When her drawings begin to affect the very fabric of reality, Amy discovers she is a rare "Threshold Child"-born during a reality shift and capable of manipulating the boundaries between dimensions.

As competing forces seek to use Amy's unique abilities for their own purposes, she finds unlikely allies in Elijah, a bookstore owner with a mysterious past, and Dr. Wilson, a scientist studying boundary mechanics. When Amy accidentally triggers a catastrophic breach between worlds, shadow entities begin slipping through the cracks into Daybridge. Now Amy must master her newfound powers to restore the proper balance between realities and become what she was born to be-not just an observer of multiple worlds, but their guardian.