Bran Yarrow loves adventuring in the great outdoors above all else. And when his mom's new marriage threatens to upend his life, it serves as the perfect opportunity to run away from home. What he didn't expect is that his hideyhole would turn into a portal to a world entirely underground!

In the world of Cavernia, where glowing mushrooms, talking bats, and crystals that can explode and shoot out energy exist, Bran is ready to finally go on the adventure he's always dreamed of. But everything is not as it seems. When a decades-old conspiracy centered around the moles controlling the planet starts falling into place, Bran teams up with Blank, a sassy, shapeshifting slime, and Altamira, a sweet girl who looks like she came straight out of hell, to save the planet from being ruled with an iron fist, find a way back home, and figure out the things that truly matter in his life.