Investigating an infestation of demons in the town of St Brett’s is the last thing that Jake Savage wants to do this summer. But for his master, the controversial Oxford scholar Roger Sotil, it is a chance to prove his theories about demons and avoid charges of heresy. The Abbot of St Brett’s has called for Roger’s help to rid his town of demons. Jake owes Roger a massive debt, but St Brett’s is a town that holds dark memories for him.

In St Brett’s Roger sees demons possessing the townspeople. Jake thinks they are just acting very strangely. The people are scared and want answers fast. A beautiful woman, Margery Haukwake, is accused of witchcraft. Roger feels sure that she isn’t guilty. Jake knows she isn’t. He was once engaged to marry her, until his widowed father stole her from him.

Margery is helping Roger with his enquiries, but she is scared. The life of her husband, Jake’s father, has been threatened by the necromancer who has summoned the demons. She knows this man’s identity but not his purpose.

Margery is convinced to testify to the Royal Justice who has recently arrived to investigate the strange events in the town. But the necromancer, a monk called Edmund Hope, has got to the Justice first, and presents evidence accusing Margery of witchcraft. Margery is swiftly put on trial, but Roger’s eloquent pleading of Margery’s case persuades the Royal Justice to free Margery and instead arrest Edmund.

When they try to arrest Edmund, the monk turns his demon-possessed minions on the Justice and kills him and his soldiers. The town, and even the Abbot, is under the control of Edmund and his demons. Roger and Jake flee the town, but Margery is captured during their escape.

Roger and Jake make their way to London to seek help. But no-one there is interested, distracted by the illness of both the King and his heir, the Black Prince. When the Black Prince dies of his illness, Joan of Kent, his widow seeks out Roger and asks him to help her son, the nine-year old Richard, heir to the throne. He has recently fallen sick and she believes he has the same disease that killed his father. She thinks that the illness is caused by witchcraft. She says that the boy’s uncle, John of Gaunt, plans to seize the throne on the death of the ageing King Edward III.

Roger and Jake realise that Edmund must be the necromancer who aids John of Gaunt.

Jake returns in secret to St Brett’s to see if Margery can be freed, while Roger stays in London to help the mother of the heir to the throne. Jake helps free Margery and those townspeople who are still free of possession. They seek refuge in a nearby castle.

Roger saves Richard from his illness with the help of a demon, Bifrons, who was previously allied to Edmund. Bifrons feels unloved by Edmund now that the necromancer has other demons to do his bidding.  

After evading demonic and mortal threats Roger, Joan and Richard, together with Bifrons in mortal guise, escape from London and meet up with Jake and Margery at the castle. Richard is furious with his uncle and vows to reveal his plot to his grandfather, Edward III. Joan sends a message to her son’s vassals and to the King to help save them from Gaunt.

Edmund has a final plan to aid his patron John of Gaunt. Instead of killing Richard, they will replace his soul with that of a demon bound to their service, and so control the kingdom. Edmund first tests this on Margery and, when this works, also takes the soul of Richard. With the help of Bifrons, Roger journeys into hell to save the lost souls, while Jake fights off Gaunt’s army with a rag-tag group of townspeople. Roger with the help of Bifrons destroys Edmund and frees the souls of Margery and Richard. On their return Gaunt realises that his plot has failed and asks his nephew Richard for forgiveness, claiming that he only sought to protect him.