Carol and Johnny Garza are 12-year-old twins whose lives in a small Texas town are forever changed by their mother’s unexplained disappearance. Shipped off to relatives in Mexico by their grieving father, the twins soon learn that their mother is a nagual, a shapeshifter, and that they have inherited her powers. In order to rescue her, they will have to descend into the Aztec underworld and face the dangers that await them.

The Garza family’s Christmas vacation in Mexico is cut short by the appearance of Pingo, one of the elfish tzapame. The news is grim: a rogue prince from an ancient undersea kingdom is seeking the Shadow Stone, a device he will use to flood the world and wipe out humanity. Now Carol and Johnny must join a group of merfolk and travel into the deepest chasms of the Pacific Ocean to stop the prince and his monstrous army with their savage magic.

When Carol and Johnny learn of the Ollamat—an ancient stone that can channel savage magic—they convince their parents to take them to the cloud forests of Oaxaca. With Pingo’s help, they search for the legendary city where it has protected for a thousand years. But the twins aren’t the only ones hunting for the Ollamat. After it is stolen, they must travel through an emerald mirror into the beautiful yet dangerous Tlalocan: the paradise of the rain god. To retrieve the stone, they have to face talking apes and forest elementals, rock wyrms and vicious elves, demons of lighting and something even more unexpected: the souls of people they have watched die. As always, they are aided by allies old and new, though nothing can quite prepare them for the biggest foe of all—a member of their very family.

Racing against the clock and against the better judgment of their parents and allies, Carol and Johnny head for the Above, the heaven of the sun god, to retrieve the lost soul of their cousin Stephanie before she becomes a cihuateotl, a fearsome ghost that haunts the evening skies. Their desperate trek lands them in the midst of a massive civil war in heaven, as a former sun god Huitzilopochtli leads rebels against Nanahuatzin Tonatiuh, trying to wrest away control of the sun so he can set the earth’s atmosphere on fire. The twins will have to forge a fast alliance among harpies, phantoms and ancient vampires if they’re going to stop the destruction of all they hold dear.