In the seaside town of Ridinghouse Bay, single mom Alice Lake discovers a man sitting on the beach outside her house. He has no name, no jacket, and no idea how he got there. Against her better judgment, she invites him inside. Meanwhile, in a London suburb, newlywed Lily Monrose grows anxious when her husband fails to return home from work one night. Soon, she receives even worse and more confounding news: according to the police, the man she married never even existed. Twenty-three years earlier, Gray and Kirsty Ross are teenagers on a summer holiday with their parents. The annual trip to Ridinghouse Bay is uneventful, until an enigmatic young man starts paying extra attention to Kirsty. Something about him makes Gray uncomfortable—and it’s not just because he’s a protective older brother