Morton promised he would take them home. And on Thursday, he keeps his promise. With Morton guiding them, Charlotte and Chase step beyond the shimmer for the first time and into the Underneath, a hidden world of tunnels, creatures, and glowing places beneath their home. Their first journey leads them to the Glow Gardens, where small keepers called Worlo tend the light that holds each section together. But when part of the garden has gone completely dark, they discover a Worlo hiding deep inside, too frightened to return. The longer it stays hidden, the more the garden begins to fail. Charlotte wants a plan. Maggie has too many. Chase is the only one paying attention. Because something about this doesn't feel broken. It feels stuck. And helping might not be as simple as bringing the light back. This is the second book in the Morton the Monster series.