Spending the summers on her family's private island off the coast of Massachusetts with her cousins and a special boy named Gat, teenaged Cadence struggles to remember what happened during her fifteenth summer.

Each summer the wealthy, seemingly perfect Sinclair family meets on their private island. Cadence, Johnny, Mirren, and Gat are a unit, especially during "summer 15," marking their fifteenth year on Beechwood-- the summer that Cady and Gat fall in love. Cady became involved in a mysterious accident, in which she sustained a blow to the head, and now suffers from debilitating migraines and memory loss. When she returns to Beechwood during summer 17, issues of guilt and blame, love and truth all come into play.