This is the story of Carbon, a young girl who lives with Sentient, a synthetic fox—her mentor, teacher, and friend. Carbon and Sentient seem to lead a very uneventful life until the day they find Scars, an old robot rusting on the outer islands of Gaea-32. The robot keeps saying "all humans to the tunnels"—which is very strange because Carbon is supposed to be the only human left on the colony. But what if it's true that other human beings are living in tunnels, somewhere on Gaea-32? Carbon will not stop until she has found those tunnels and the people living underground. The truth, however, might turn out to be quite different from what she expected—or even ever imagined.