They were the most prominent American family of the 20th century. But one daughter remained hidden. Joe and Rose Kennedy's strikingly beautiful daughter Rosemary attended exclusive schools, was presented as a debutante to the Queen of England, and traveled the world with her high-spirited sisters. And yet, Rosemary was intellectually disabled—a secret fiercely guarded by her powerful and glamorous family. Major new sources—Rose Kennedy's diaries and letters, school and medical records, and exclusive family interviews—bring Rosemary alive as a girl adored but left far behind by her competitive siblings. Kate Larson reveals both the sensitive care Rose and Joe gave to Rosemary and then—as the family's standing reached an apex—the often desperate and duplicitous arrangements the Kennedys made to keep her away from home as she became increasingly intractable.