Twelve-year-old Elias has consumption, so he is sent to Kentucky's Mammoth Cave--the biggest cave in America--where the cool cave vapors are said to be healing. At first, living in a cave sounds like an adventure, but after a few days, Elias feels more sick of boredom than his illness. So he is thrilled when Stephen, one of the slaves who works in the cave, invites him to walk further through its depths.

But there are more than just tunnels and stalagmites waiting to be discovered; there are mysteries hiding around every turn. The truths they conceal are far more stunning than anything Elias could ever have imagined, and he finds himself caught right in the middle of it all--while he's supposed to be resting. But how can he focus on saving his own life when so many others are in danger?