The new fantasy novel Demon Tree commences in the near future. Society has been decimated during the Fall, when strange lights in the sky led to widespread madness across the Northern Hemisphere and the building of the Great Wall. Soon after grew the Cazash Tree. Starting as a forgotten seed in the London Natural History Museum before overwhelming the ruined city, the Tree reached upwards hundreds of feet to create a twilight world. The Tree is endlessly inventive and it learns fast. It has started to create other entities from the living wood, including the disciples – thirty-foot high creatures that roam the surrounding forests – and a cunning and cruel homunculus named Darkness. The sporing is now approaching, and time is running out for life on Earth. Science has failed to stop the Tree, but there are more ancient and earthly powers that are awakening. Nature goddess Annan has been forgotten since Celtic times, but the Cazash Tree is not of her making and she is angry. When she awakens druidic priest Myrddin Wyllt and tells him what he must do, a sequence of events commences that changes everything.
The city’s survivors on the streets – bough-runners – are nourished by kine, an addictive sap that bestows gifts upon its servants and terminates those no longer useful. Seventeen-year-old Swift is a bough-runner, and like the others has been given a special gift from the museum by the Tree. Swift inherits a museum-piece Colt 45 and the lightning skills of a gunslinger from her lover Lonewolf when his time comes, and she must use the gun to end his suffering. Swift is different from her kind, determined to find the mother she dimly remembers and to revenge the death of her lover. She vows to kill the Cazash Tree but cannot do it alone.
Below the streets, deep in the London Underground tunnels live the Hellon. Invisible to the Cazash Tree, they dwell in total darkness and have surrendered the gift of sight. Talented engineers, the Hellon are destined to play a key role when Swift joins them.
Scattered groups of nature-loving Edenists inhabit the vast forests beyond the city. Amongst them are relict scientists from before the Fall, desperately seeking a way to prevent the spawning, and there is growing tension between the sects. Some Edenists possess second sight, but Bella Oak has much greater powers and her autistic younger sister Hope is stronger still. Bella and Hope’s parents are scientists, but the girls are also children of the forest. Whilst bough-runners are hated by Bella’s people, she and Hope are destined to join Swift in the fight for freedom after the killing of their parents.
Bough-runner Carmen Ghoul is mother to Swift, although they have been separated since being chosen. The Tree senses a new threat buried deep on a hillside bearing a White Horse, where a strange creature named Myrddin Wyllt is awakening. Carmen travels westwards on her own at the behest of the Tree, but is equally drawn by the new force.
The giant Myrddin awakens in a crypt below the Downs. His last memory is of having his throat slit by a priest named Father Bartholomew, having been betrayed by sorceress Hama. Re-born and not entirely happy, the wizard escapes from his tomb below the White Horse and heads for the forest. Soon after, he encounters Bella and Hope Oak facing a disciple. He is unable to save their father and is stung and blinded in the process, but also witnesses the psychic powers of the two girls as Hope sets fire to their attacker. As Bella and Hope care for him, the child-like Myrddin discovers he has been dead for fifteen hundred years and the world is much changed. He learns of the Cazash Tree and understands why he has been awakened by the goddess Annan. He must find seven warriors to assist him in his quest, in a world that has become fragmented and confused. Hope is amongst them, also Cerebos the hellhound and Hama the sorceress. Myrddin summons Cerebos and awakens Hama, but summoning her will require much stronger magic and he does not relish the prospect. The secret of how to do so is trapped somewhere in the past and getting there will not be easy.
Hama was hoping to become wraith and to escape the torment of the flesh with her beloved Dra-Gon, and is sad at being awakened by Myrddin, who she once loved but learned to hate. She knows he needs her help, but why save a world that has no place for her kind? Hama decides to complete her journey with the only one who understands her, but Dra-Gon is buried in the Swiss Alps below a magical obelisk, and Hama lives south of the Great Wall. Managing to ignore Myrddin, Hama sets off on her quest.
The druidic priest does not understand the world he has awakened in. He can seem primitive and uncaring but develops a strong bond with the girls. He longs to find Hama, but also fears what she might do. When Bella informs the people of her village that Myrddin has awakened, the high priest Artur Blackstone sees it as a threat and accuses her of blasphemy.
Myrddin recovers and helps Bella escape the members of her village, but Hope hides in the forest. Bella explains that there is a sickness amongst the villages, and bodies have been found with empty skulls surrounded by a grey powder. The victims have been infected by the Tree and are bearers of male spores, and Artur Blackstone has become a servant of the Cazash Tree.
Carmen Ghoul tracks down Myrddin and Bella. Myrddin is able to free Carmen from the hold of the Cazash Tree. Carmen and her daughter have been chosen to help him. Carmen sets off for London to find her daughter, and Bella leaves to find Hope. Myrddin heads east towards London in search of a warrior named Gwynn De Vere.
Fortified towns hide within the oak forests, each a despotic kingdom competing for resources. Avalon is one such town, a chaotic jumble of buildings upon buildings dominated by a giant red crane. Avalon, once known as Guild Ford, is ruled by the De Veres, a despotic family intent on building a rival empire to the Cazash Tree. The De Veres want the powers of the Edenists, and their matriarch Gwynn experiments on gifted children using a derivative of kine. She is one of the heroes, unlikely though it seems.
Myrddin arrives at Avalon during an execution and witnesses the widow and children of the dead man leap from their roof, falling to the street below. He discovers that the widow and son are still alive but the daughter has vanished. He saves the boy and helps the mother, named Ruin, escape. Ruin takes him to their simple house high up in the town, where he finds out that Dock is as gifted as Hope. Myrddin meanwhile makes himself known to Gwynn and tells her of his quest – that to defeat the Tree, he must summon Hama, and the magic to perform the summoning is somewhere in London but far back in time. She sends three of her knights with him – Lem, Jenk and the female warrior Loner.
Bella eventually finds Hope, but the girl is suffering from exposure. Bella helps overcome her trauma before heading back to their village. They discover something is wrong and hide in the forest where they can watch. Many villagers have died and Blackstone is having the bodies buried in a mass grave. Their leader has become more tree-like and the surviving villagers are becoming like him. He detects the presence of the girls and soon after, the roots of the Cazash Tree force their way through the forest and disciples are not far behind. As the sun sets, the girls are surrounded until Hope uses her powers and the village is destroyed. The girls escape, heading onto the Downs and away from the forest, which is no longer safe for them. Two disciples are already in pursuit when a hound the size of a bear races towards them, with a red light in its jaws. The disciples are torn apart by Cerebos, which soon leaves and heads for the ruined city in search of its master. Bella and Hope follow until the Cazash Tree traps them in a cage made of roots, and Blackstone arrives. The Cazash Tree murders Bella but before it can kill Hope, she discovers her power again and this time, the roots and Blackstone are consumed by fire. Dazed and distraught, Hope makes for Avalon where she is imprisoned with the other gifted children. Dock is there, and another girl named Anna. Dock has powers as great as Hope, but fear makes both of them weak.
Still overwhelmed by sadness, Swift makes her way to the heart of the Cazash Tree. Not knowing what to do, she faces several disciples and the homunculus named Darkness. Her death is certain until Cerebos appears. He takes Swift into the underground world of the Hellon before leaving her, and she is eventually discovered and invited to join, but must first survive a trial which leaves her much changed, and her skills are intensified.
Carmen Ghoul is lost in the tunnels below the city and resigned to dying from thirst, when she wanders into a confrontation between scavengers from one of the fortified towns and three Hellon. One of the towners carries a gun, whereas the Hellon are armed with Samurai swords. The girl with them draws and fires a heavy Colt, and the fight is soon over. Only then does Carmen recognise her daughter and is taken back with them into the darkness.
Myrddin’s journey with the three knights into the heart of London is filled with danger. They reach Surbiton and must hide from a new form of disciple that can fly. They move from house to house, trying to evade detection and Myrddin discovers that the flying creatures, which resemble pterosaurs, are able to breed independently, and hunt other creations of the Tree. It is creating a replacement ecosystem, and it is not long before Lem and then Jenk are killed. Myrddin tries to protect Loner from further threats as they near the museum, when an attack from above kills her. Myrddin transforms into a flock of starlings and escapes, finding his way to the place where the magic is coming from. Once inside, he discovers a hidden door that opens into the past. He finds himself in the castle where he was raised as the child of a servant, but this version of the past is different from what he remembers. He encounters an aged version of himself who provides him with the books he needs to summon Hama.
In Avalon, Hope and Dock witness the death of Anna and fear gives them strength. Hope is finally able to use her powers when needed and escapes with Dock, telling him they must follow Cerebos the hellhound and head into London, and he agrees. Soon after they have gone, one of the pterosaurs discovers the town. Other curious creatures are making their way through the forest, including huge club-tailed beasts, and a strange creature designed for carrying living saw-disks. Before long, the town is under siege and the red crane is felled. Despite valiant efforts, the town is lost and Gwynne narrowly escapes. Eventually, the Cazash Tree calls off its forces and the survivors are able to emerge. Gwynne knows the Tree was searching for Myrddin. Disheartened, she leaves Halo and makes her way to the stone table, drawn by its power.
Hama has suffered much since her awakening and her sorcery is much diminished. She finds a way through the great wall and sees many terrible things, and piles of bones reach halfway up the northern side. She obtains money by selling herself but is forced to kill the man to save herself. She finds a drug-running pilot who takes her as far as Cyprus before crash-landing. He tries to take her captive, and so she kills him too. Finally, she finds the captain of a hydrofoil who says he will take her to Venice, but he is a DeVere and a slave-trader, searching for people with special gifts. Hama is chained to the other captives, until there is a confrontation and she breaks free. The slaves take over the ship whilst Hama fights with the captain, but they are out of control and the hydrofoil heads inland through the lagoons before crashing and burning. Hama is badly injured but she cannot die. She makes her way northward to Switzerland to find the obelisk as her body repairs, and all the while the summoning grows stronger. She knows Annan is calling upon her to help Myrddin, but she stubbornly refuses.
The druid meanwhile returns alone with the magic book, but encounters Hope and Dock heading for London with a bough-runner named Willow. He kills her before she can reveal their location, and they go in search of Cerebos and the other heroes. Once in London, they make their way into the Underground tunnels from Heathrow Airport, and are eventually reunited with Swift and Carmen, and Cerebos. Now they are five.
Myrddin deciphers the book. Three rune-stones must be found, and the clues in the book suggest that one is in the old Channel Tunnel, another is at a site in Hungary, not far from the terminus of the underground railway. The third is hidden in the museum, below the Cazash Tree itself. Touching a rune-stone will send it back to the mother-stone below the sacrificial table on the Downs, but the touch is fatal. Myrddin, Cerebos, Carmen and Swift set off on an epic train ride with driver Thomas, with the knowledge that three of them will die. They encounter resistance on their journey from renegade bands of Hellon, but they are no match for the heroes. When they find the first rune-stone, Thomas sacrifices himself and the stone is secure. Hope and Dock enter the museum building below the Cazash Tree and face many dangers, but finally locate the stone. They touch it together, hoping that their powers will save them, but Dock is lost. Carmen gives her life for the third stone, repaying her debt. Swift drives the train back to London where a Hellon army awaits.
Hama continues her journey, drawn by the power of the obelisk and her longing for peace, but she senses the power of the Summoning growing stronger. Eventually she finds the obelisk. She utters the words and waits, hating herself for abandoning Myrddin.
Swift and Myrddin return to the stone table with the Hellons, and on their way discover hundreds of infected people standing on top of buildings, planes and other high points, waiting for the moment to spore. The Tree has sent up more of the giant flowers and only the endless rain keeps it at bay, but even Myrddin cannot make it rain forever.
Many thousands of the infected stand waiting for the rain to end, wherever the wind is strong. The giant flower stalks face to the West and the forests are silent. Myrddin and Cerebos arrive with Swift, and meet with Hope and Gwynn, but they are only five. Hama and Dock are missing. Emboldened, the Tree sends thousands of its servants to attack. But Myrddin can now use the symbols to summon Hama.
Hama waits beside the obelisk. She believes she has failed when she feels the ground below her heave upwards before tearing open. She watches as her beloved Dra-Gon emerges. The body is the size of a jumbo jet and so is each wing. Its tail is a hundred yards long and the pupil of each eye is taller than a man. Dra-Gon is not pleased at the awakening. The world has changed, and every step and each breath are agonising. But Dra-Gon has been awakened by its mistress, and it tells Hama to climb into its ear before it begins the journey to England to save Myrddin.
The rain has almost stopped. The sides of the Downs are thick with the fallen, yet still more disciples, homunculi and saw-disks are coming. The silvery roots of the Cazash Tree are attacking from all sides when there is a sudden light from the stone table and when the light fades, Dock is there. He holds hands with Hope and they use the Push together. This time it is stronger than ever, and the immediate attack is ended. The demon tree seems to be boiling with rage and spores are streaming from the giant flowers.
The countless servants of the Tree are no match for dragon-fire, but Dra-Gon leaves Hama on the battlefield and heads to London. Hama joins the fight and is reunited with Myrddin, and they embrace one another. As the demon tree tries to spore, the dragon attacks repeatedly until the city is on fire. In a final act of defiance, Dra-Gon dives into the Tree and all are consumed in the inferno as the dragon becomes wraith and is finally free. Hama and Myrddin cannot return to their time, and must find a way back from hate to love.
Swift, Hope and Dock must together help build a new world, but people are hard to change. A little while later, a new sect begins to form, worshipping a fragment of the demon tree which is already growing roots. A self-appointed priest is preaching intolerance and hatred, refusing to recognise the secular way of life. Judgement is given and as the novel ends, the sound of a Colt .45 echoes from the hills.