When Cara and Brendan first meet, she's fresh out of college with a degree in the fine arts, 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 claims--no, he insists--that he he can breathe underwater. He shows Cara his gills. He dives beneath the waves and doesn't emerge for many minutes at a time. He offers her the most plausible of explanations: that he is a member of the United State's Army Special Forces and has undergone top-secret experimental surgery. And Cara, struck by the force of his devotion, by his unstoppable charisma, and most of all, by the casual truth of his claim, believes him. Their summer romance quickly turns serious. And then Cara gets pregnant. She and Brendan move into a house he buys for them, and when their son, Micah, is born, she is sure their happy ending is underway. Still, she is forced to contend with Brendan's dramatic moods, and struggles to overlook his unexplained disappearances and the weight of his dangerous secrets. She knows it must be PTSD. The trauma of war. The desperate, tragic memories that scar all soldiers. Cara is determined to stay strong for her young family, to heal Brendan's psychic wounds, to keep him safe. Until he and baby Micah seemingly vanish into thin air--or deep water. 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 a man and small child treading water in Nantucket Sound. The news rekindles Cara's never-abandoned hope that her child may still be alive. As she fights to untangle delusion from reality, and revisits a past she's worked hard to reconcile, she's determined to learn the truth about her lost love and finally find her son.