Mrs Cornelia van Doorn, a wealthy South African widow, has leased Castle Gloaming intending to launch herself and her stepdaughter, Honor, into British society. The castle stands in rugged Welsh countryside, overlooking the foaming River Dee. Its hills are haunted by tales of mythic creatures, including the ghost-dog, Gelert. Its most recent owner, the eighth Earl of Barhaven, has died and Castle Gloaming needs money, which Mrs van Doorn appears to have in abundance.

Dressmaker-sleuth Jemima Flowerday is hired to dress Mrs van Doorn and Honor in the privacy of Castle Gloaming. Except that Honor is nowhere to be found and Jemima is asked to investigate. The nineteen-year-old seems to have melted into thin air....