A tiny girl is abandoned on a ship headed for Australia in 1913. She arrives completely alone with nothing but a small suitcase containing a few clothes and a single book -- a beautiful volume of fairy tales. She is taken in by the dockmaster and his wife and raised as their own. On her twenty-first birthday they tell her the truth, and with her sense of self shattered and with very little to go on, "Nell" sets out on a journey to England to try to trace her story, to find her real identity.
Her quest leads her to Blackhurst Manor on the Cornish coast and the secrets of the doomed Mountrachet family. But it is not until her granddaughter, Cassandra, takes up the search after Nell's death that all the pieces of the puzzle are assembled. At Cliff Cottage, on the grounds of Blackhurst Manor, Cassandra discovers the forgotten garden of the book's title and is able to unlock the secrets of the beautiful book of fairy tales.