After a world-changing pollution wave sends people below ground, Elias is the only human still hanging around Mexico City. That is, until he meets Ana. She has snatched a breathing suit from her tunnel home and come searching for a way to clean the air. When the two collide, Ana will get an up-close look at the city's mutated creatures. And Elias will discover the secret of how he breathes above ground. Together, the kids will need each other-and maybe even some creatures-to learn if a better world is possible. But first, they'll have to avoid the clutches of the fearsome Rat King, who's out to take them both prisoner.