When Cara and Brendan first meet, she's fresh out of college, recovering from the recent death of her mother, and spending time on Martha's Vineyard while trying to figure out her next steps. She's swept away by Brendan's humor and charm, and intoxicated by his thrilling, dangerous secret: he can breathe underwater. Able to stay beneath the waves for longer than should be possible, Brendan reveals that he is part of a secret experimental unit of the US Special Forces. And Cara, struck by the power of his conviction, by his unstoppable charisma, and by the evidence before her, believes him.

Their summer romance turns serious. Then Cara gets pregnant. When their son, Micah, is born, she's sure their happy ending is underway. Still, she's thrown by Brendan's dramatic moods, his unexplained disappearances, and the weight of his secrets. Cara is determined to stay strong for her young family--until he and baby Micah vanish, leaving her desolate and alone and questioning everything she once thought was true.

Five years later, Cara is still struggling to move forward, married to another man and trying to rebuild her life, when a local fisherman announces he's spotted two people--one of them a small child--treading water in Nantucket Sound, far from any vessels and miles from shore. The news rekindles Cara's never-abandoned hope that her little boy may still be alive. As she fights to untangle delusion from reality, and revisits a past she's worked hard to reconcile, Cara is determined to learn the truth about her lost love and finally find her son in this "book you won't be able to put down" (Jessica Anya Blau, author of Mary Jane).