Glastonbury, legendary resting place of the Holy Grail, is a mysterious and haunting town. But when plump, dizzy Diane Flitch returns home, it's with a sense of deep unease — and not only about her aristocratic family's reaction to her broken engagement and her New Age companions.

Plans for a new motorway have intensified the old bitterness between the local people and the 'pilgrims', so already the sacred air is soured. And, as the town becomes increasingly split by violence and death, Diane, local bookseller Juanita Carey and the writer Joe Powys must now face up to the worst of all possibilities: the existence of an anti-Grail — the dark chalice.