The country of Avilonía is a land ruled by warriors who were blessed by elemental gods during a time of colonization and war. In the coastal city of Coáraluna, capital of the Moon Kingdom, lives Esmé Vespertín—a shy refugee who takes a potion for her migraines that cause her to see strange shadows. On the outside, she is a normal girl trying to get by in a world that could crumble at any moment. Yet, she harbors one of the biggest secrets in all of Coáraluna: she’s been entertaining a secret relationship with none other than Río Markaél – Avilonía’s Moon Prince. 

Principe Río is a tide bender who is trying to prove himself by going on secret missions to help his father on the war front. His latest quest was the most ambitious yet, leaving him half-alive with a cursed object that could change the course of the ongoing war forever. However, when Esmé finds it by accident one evening, things take a turn, and on the night of the yearly Winter Solstice Ball, an attack on the palace reveals that she may be more special—or terrifying—than anyone could have imagined.