In modern-day Mexico, Sitlali is all alone after the death of her beloved abuela. Targeted by a dangerous gang member, she flees to the United States to find her father. The night before her journey, she finds an ancient conch shell on the beach and takes it with her as a memento of home.

In 1521, Calizto is trapped in the Aztec city of Tenochtitlan, which is besieged by Spanish invaders. He has fought valiantly, but hope for his people is running out. Desperate to escape, he takes up his mother's sacred conch and sounds a plea to the gods.

The conch holds magic neither Sitlali nor Calizto understand, magic that allows them to communicate across centuries--and find comfort in each other as they fight to survive. With each conversation, they fall deeper in love, and as the moon waxes, they become more present to each other. But as danger threatens at every turn, will they ever find a way to truly be together?