There is often trouble of a mythical sort in Bath. The booksellers who police the Old World keep a careful watch there, particularly on the entity who inhabits the ancient hot spring. Yet this time it is not from Sulis Minerva that trouble starts. It comes from the discovery of a sorcerous map, leading left-handed bookseller Merlin into great danger. A desperate rescue is attempted by his sister the right-handed bookseller Vivien and their friend, art student Susan Arkshaw, who is still struggling to deal with her own recently discovered magical heritage.

The map takes the trio to a place separated from this world, maintained by deadly sorcery performed by an Ancient Sovereign and guarded by monstrous living statues of Purbeck marble. But this is only the beginning, as the booksellers investigate centuries of disappearances and deaths and try to unravel the secrets of the murderous Lady of Stone, a serial killer of awesome powers.

If they do not stop her, she will soon kill again. And this time, her target is not an ordinary mortal.