Lexi Laney and Clare Hart grew up together having wondrous adventures across picturesque Nantucket. And when it was time to share intimate secrets and let their imaginations run free, they escaped to their private hideaway: Moon Shell Beach. But nothing stays the same. With the complicated pressures of adulthood, their intense bond is frayed, hurtful words are exchanged, and Lexi flees Nantucket to a life of luxury while Clare stays behind.

Ten years later, a newly divorced Lexi returns to make amends with those she left in her wake-particularly Clare, who still simmers with resentment toward her friend. Their emotional reunion is beset with major challenges, as Lexi's return sets off a series of startling events. And as Clare's life takes an abrupt detour, Lexi wonders if the happiness and peace they once knew on Moon Shell Beach will, in the end, prove to be as fleeting as time and the tide.