After a two-day train journey, peripatetic dressmaker Jemima Flowerday makes the final approach to her new job on foot, following the canal to Castle Gloaming, which stands in the rugged North Wales countryside overlooking the foaming River Dee.

Jemima has come to clothe Mrs Cornelia van Doorn, a wealthy South African widow who has leased the castle from its impoverished aristocratic owner, intending to launch herself and her young stepdaughter, Honor, into London society. But on her arrival she's told she's actually been summoned to investigate the disappearance of the girl. Even in a place still haunted by mystery and magic, the seventeen-year-old can't have melted into thin air…