In the summer of 1977, Victoria Leonard's world changed forever — when Caitlin Somers chose her as a friend. Dazzling, reckless Caitlin welcomed Vix into the heart of her sprawling, eccentric family, opening doors to a world of unimaginable privilege, sweeping her away to vacations on Martha's Vineyard, a magical, wind-blown island where two friends became summer sisters....

Now, years later, Vix is working in New York City. Caitlin is getting married on the Vineyard. And the early magic of their long, complicated friendship has faded. But Caitlin has begged Vix to come to her wedding, to be her maid of honor. And Vix knows that she will go — for the friend whose casual betrayals she remembers all too well. Because Vix wants to understand what happened during that last shattering summer. And, after all these years, she needs to know why her best friend — her summer sister — still has the power to break her heart....