A beautiful discovery outside the window of her underwater home prompts the reclusive E. to begin a correspondence with renowned scholar Henerey Clel. The letters they share are filled with passion, at first for their mutual interests, and then, inevitably, for each other. Together, they uncover a mystery from the unknown depths, destined to transform the underwater world they both equally fear and love. But by no mere coincidence, a seaquake destroys E.'s home, and she and Henerey vanish. A year later, E.'s sister Sophy, and Henerey's brother Vyerin, are left to solve the mystery, piecing together the letters, sketches and field notes left behind—and learn what their siblings’ disappearance might mean for life as they know it.