For FBI handler Meg Jennings and her K-9 partner, Hawk, the search for a missing girl leads to the underground caves of Kentucky—ones darker than she ever fathomed.

No descent into the unknown has ever been too treacherous for Meg Jennings and her colleagues in the FBI K-9 unit. They are prepared for anything. But when 14-year old Larissa Whorlow disappears on her way home from school in Nashville, Tennessee, Meg may be proved wrong.

Their only hope is a tracker tag on Larissa’s shoe, worn at her protective mother’s insistence. Unfortunately, the trail stops dead at Mammoth Cave National Park in Kentucky. Meg and her black Labrador, Hawk, are among four teams scrambling to solve a kidnapping that has crossed state lines. All Meg can do is follow Hawk, who’s on the scent of the missing girl—right through a narrow break in a limestone rise and straight into a cavernous labyrinth.

With all communication cut off and miles of undiscovered passages and subterranean rivers to navigate, Meg’s search gets deeper and darker with every uncertain move forward. Then, at the edge of a dangerous abyss, Meg makes a shocking discovery and realizes that Larissa’s vanishing was no random snatch and grab. It’s the work of a serial killer who’s pulling Meg into a terrifying game of hide-and-seek. And the only way out is through the blackness below.