Nine-year-old twins, Emma and Martín, couldn't be more different in their personalities, interests, and even their looks. But one thing they absolutely agree on is that moving from Cuernavaca, Mexico, to Illinois is a terrible idea. Unfortunately, they're not given a choice when their dad lands his dream job as a middle school principal in Chicago. To help the twins stay connected to their Mexican heritage, their abuela gifts them a book of Mexican legends. The book turns out to be more than a going away present...it's a magical item that transports them directly into the legends!

In the first legend, Emma and Martín encounter Tlaloc, the god of rain. Tlaloc is angry because his lightning bolt has been stolen, and his rage is manifesting as a torrential downpour over the ancient city of Texcoco. The rain won't stop until the lightning bolt has been returned, so Emma and Martín set out to recover it.

Will they find Tlaloc's bolt in time to help the people of Texcoco save their home? Or will the wrath of the rain god mark the end of this legendary city?