When a vivid prophetic dream wakes Ariadna in the middle of the night, she and Percy are summoned to the Oracle and given a cryptic quest about a lost treasure tied to the “mother of nymphs.” Forced to leave the safety of Camp Half-Blood behind, the two demigods must follow fragmented visions of a mysterious island where sun and sea must unite to recover what has been lost — unaware that this mission may change the course of their summer, and perhaps far more than that.