When high school senior Noah Gallagher and his adopted teenage sister, Lo, go to live with their grandmother in her island cottage for the summer, they don't expect much in the way of adventure. Noah has landed a marine biology internship, and Lo wants to draw and paint, perhaps even to vanquish her struggles with bulimia. But then things take a dramatic turn for them both when Noah mistakenly tries to save a mysterious girl from drowning. This dreamlike, suspenseful story—deftly told from multiple points of view—dives deeply into selkie folklore while examining the fluid nature of love and family.