Middle-schooler by day. Monster hunter by (k)night. Lucia Castillo dreams of being a hero like her grandfather. But to their Texas town, he was just a strange old man who dressed up as a knight and claimed to save the world from monsters only he could see. Now years later, when Lucia and her best friend—and trusty squire—Sandro discover the town mayor is secretly a shapeshifting beast of Mexican lore, her parents think she's imagining things like her "Abuelo Loco." Only Lucia, wearing her grandfather’s magical helmet, can see the hidden threat. That is, when she's not in detention or grounded. Can she and Sandro prove the others wrong and stop the mayor from unleashing evil on their town—and beyond?