Underground. Underwater. Out of time. Ollie had only wanted to make things better at Herrick's End. And he thought he had, until he sees the stark truth spelled out in black-and-white: His friends are in danger, and it's all his fault. The good news? There might be a solution. The less-good news? It's hidden at the bottom of a deep, dangerous lake. Leaping into that water, he knows, is a monstrously bad idea. It's also the only idea he's got. One thing is certain: Ollie's quest to right past wrongs is about to open up a whole new can of wormwalkers in the extraordinary underground world he now calls home.