Young Maeve is disgruntled and depressed when she is sent for the summer to her aunt's house in Newfoundland while her parents try to work out problems in their marriage and careers. But she does enjoy Newfoundland's rustic charms and the closeness she feels there to her favourite grandmother, now dead. Maeve's grandmother was the author of a fantastic book about the secret world of Annwn and Maeve is delighted to discover her grandmother's diary and a beautiful Celtic brooch that was stored with it.

Then something strange begins to happen, something that convinces Maeve she is leaving Newfoundland, and in no ordinary fashion. Before long, Maeve finds herself in the Hidden World, a place she is sure her grandmother has visited too, a place that is alive with magic and danger. As she struggles to help her new-found friends in Annwn and unravel the mystery of her grandmother's time there, she finds herself immersed in a world of legend, where Celtic myths, tales of the sea and the secrets of Avalon come together in a fantastic and horrifying way.