The protagonist Peter Grant is left shaken by the developments at the end of the previous book, the sudden betrayal and defection by a highly valued colleague to whom Grant also had a strong emotional tie. The moping Grant welcomes the chance to leave the familiar grounds of London and travel to rural Herefordshire, where the disappearance of two eleven-year old girls is a media sensation, the focus of an intensive police search - and might have grave magical implications as well.

Grant finds that the tangle of marital and extra-marital relations in a small rural community is not only a matter for gossip, but bears very serious criminal implications, and some supernatural ones as well. He meets with a retired wizard, traumatized by the secret magical battles of World War II, and with the wizard's granddaughter who has a very special affinity with bees. Grant gets into intensive contact with Beverley Brook, the goddess or Genius loci of Beverley Brook, a tributary of the Thames - and learns by personal experience just how rivers gain such gods. He finds that unicorns are all too real and that their horns are deadly weapons; that fairies do exist and even in the 21st century they do sometimes kidnap human children and replace them with changelings; and he meets with a real-life faerie queen, very different from the one imagined by Spenser.

As the ultimate result of all that, Grant faces the prospect of being stuck forever as a captive in the real-life fairyland - an alternative reality or Otherworld where Britain is still covered with a massive unbroken primeval forest, with no sign of the familiar towns and villages. Grant's single, slender hope of escape lies in the lasting magical (or possibly anti-magical) effect of the Roman Empire's engineering projects and of the Romans' habit of imposing themselves on the landscape and building "roads straight as an arrow" wherever they ruled.

Returning characters edit

• Police Constable Peter Grant; an officer in the Metropolitan Police and the first official apprentice wizard in sixty years.
• Lesley May; formerly Police Constable in the Metropolitan Police and de facto apprentice to Nightingale; now criminal associate of the faceless man and subject to an internal investigation
• Detective Chief Inspector Thomas Nightingale; head of the Folly and the last officially sanctioned English Wizard.
• Molly; The Folly's domestic helper, of unknown species.
• Dr Abdul Haqq Walid; world-renowned gastroenterologist and cryptopathologist.
• Beverley Brook; goddess of a small suburban river and consulting ecologist.

Characters introduced in this novel edit

• Detective Constable Dominic Croft; a member of the West Mercia Major Crimes Unit assigned as Peter's liaison.