Anna lives with foster parents, a misfit with no friends, always on the outside of things. Then she is sent to Norfolk to stay with old Mr and Mrs Pegg, where she runs wild on the sand dunes and around the water. There is a house, the Marsh House, which she feels she recognises - and she soon meets a strange little girl called Marnie, who becomes Anna's first ever friend. Then one day, Marnie vanishes. A new family, the Lindsays, move into the Marsh House. Having learnt so much from Marnie about friendship, Anna makes firm friends with the Lindsays - and learns some strange truths about Marnie, who was not all she seemed...