Jack Chatwin has visions, which leave tangible evidence - sounds and smells, which linger afterwards. What he sees are two primitive figures, with painted faces - Greyface and Greenface, a brother and sister. He calls them bullrunners.
John Garth is a city dowser, searching for the mythical pre-Roman city of Glanum. He hopes to find an entryway to the elusive city beneath Exburgh, Jack's home town. And he thinks Jack's bullrunners may be connected to Glanum . . .
Years later, Jack, now grown up, agrees to take part in experiments to investigate his bullrunners - until Greyface, the male, breaks free of Jack and takes corporeal form. The bullrunner kidnaps Jack's young daughter so Jack will force Greenface to follow her brother-husband, even against her own wishes. Though Greyface returns the daughter, he keeps a shadow of her, which takes on a life of its own. If Jack refuses to co-operate, the shadow will drain his daughter's vitality and personality - and her very future.

The story of Jack's search for Greenface is interwoven with the connections between the bullrunners and the mystical city of Glanum in this resonant tale of ancient mythic wonder.