On a world colonized long ago by humans from Earth, the descendants of those colonists have lost their technological heritage but retain a mythologized memory of their origins, interpreting the remnants of their ancestors' science as magic and legend. Lynesse Fourth Daughter, the youngest child of the Queen of Lannesite, grows up on tales of her great-grandmother Astresse Once Regent, who rode to war alongside the ancient sorcerer Nyrgoth Elder. When a mysterious force in the Ordwood, a forested region of small kingdoms, begins corrupting the land and enslaving the minds of people and animals, the queen dismisses the reports as civil strife. Lyn, acting without royal permission, sets out with Esha Free Mark, a traveler and diplomat of the Coast-people, a semi-autonomous ethnic group whose ancestors were bioengineered for aquatic life. Together they climb the forbidden mountain to the Elder's tower, where a disembodied voice authenticates Lyn's royal bloodline and admits them.
The narrative shifts to Nyr Illim Tevitch, an anthropologist from Earth's Explorer Corps stationed on the planet Sophos 4 to study the colonists' descendants. His colleagues left long ago, and he has received no communication from Earth in 291 years. Waking from cryogenic suspension, he is overwhelmed by depression and activates his Dissociative Cognition System (DCS), an augmentation that compartmentalizes his emotions so he can function rationally. He mistakes Lyn for Astresse, with whom he fought side by side generations earlier, and learning Astresse has been dead for over a century devastates him.
From Lyn's perspective, the Elder is a seven-foot, horned, gaunt figure in robes glittering with golden sigils. He dismisses her plea, but when she describes a power in the Ordwood that steals minds and cannot be fought with blades, Nyr recognizes he has no remaining purpose and no one coming for him. He agrees to go.
They travel by hidden passes. A malfunctioning mining robot, a centuries-old relic of Nyr's mission, attacks them and destroys Lyn's sword before Nyr pacifies it with verbal commands. That evening, Nyr tries to describe his clinical depression as a beast that hounds him, but Lyn interprets this as a literal predator, capturing the persistent gap between their worldviews: what he calls science, her culture understands as sorcery. At the riverside town of Wherryover, locals attempt to saw off Nyr's horns in the night. His defensive clothing injures them with a burst of heat, electricity, and radiation, and the crowd interprets his clinical explanation of their injuries as a sorcerer's curse.
Recognizing his DCS has been overtaxed, Nyr disables it alone one night and is hit by a cascade of suppressed emotion: despair over Astresse's death, the futility of centuries of work no one will read, his total isolation. He collapses sobbing. In the morning he finds a blanket over him; Lyn and Esha watch through the night, swords drawn, believing a real beast is stalking him.
At Watacha, a walled Ordwood city overflowing with refugees, petitioners at a formal assembly describe the demon's effects: orchards bearing unnatural moving fruit, people and animals infected with scaly growths and bead-like sense organs with their wills enslaved. Allwerith Exiled, a man whose status-title marks his banishment and who lost his fingers as part of his criminal punishment, gives an eyewitness account of the demon's origin near Farbourand, where organic structures grow from the earth incorporating living beings. Lyn delivers a rousing speech declaring Nyr will destroy the demon. Nyr watches in silent distress.
Nyr attempts to explain the true colonial history: generation ships from Earth, economic collapse, the gradual loss of technology. His scientific account and the women's mythology map perfectly onto each other, and they respond simply that yes, this is how they tell it. When Nyr later shouts that there is no magic, Esha counters that every magician studied how the world worked to control it; his science and their magic differ only in degree.
Allwerith, now pardoned and called Allwer, guides them west into corrupted territory. They encounter rodent-like creatures fused into a towering living mass emitting electromagnetic signals that defy known physics, and at the half-burned town of Birchari, demon-slaves including a mutated human. Nyr discovers he can jam the demon's signal by broadcasting on its frequencies, freezing its thralls. But a hidden appendage inside the corrupted human impales him through the gut. His internal systems nearly trigger the satellite's Ultimate Anti-contamination Measures, an orbital strike designed to obliterate him and all nearby technology to prevent cultural contamination, but he barely countermands the order.
Nyr wakes two hours later; the others assume he is dead. Esha has been infected during the fight, but Nyr extends his protective field to cut her off from the demon's signal, and Lyn and Allwer remove the limp growths. Nyr confesses that had he died, fire from heaven would have destroyed everything nearby. Lyn grants him the right to use her familiar name, a significant cultural concession.
That evening near the demon's lair, Nyr asks to lower his emotional defenses one last time. Lyn arranges a night of storytelling. Nyr tells the story of a man from the otherworld whose companions left and whose people fell silent. Lyn tells the legend of Astresse. Nyr weeps and smiles, saying her version is better than the truth. He loved Astresse, and for their compact he will destroy the demon if he can.
Nyr deploys a drone to survey the infection's center and discovers thick organic tendrils grown into an arch roughly ten meters tall, a portal opening onto a landscape of impossible colors and dimensions. He concludes the demon may itself be the signal, a non-physical entity causing catastrophic changes in living matter, with scattered growths serving as relay stations for transmissions traveling through some medium outside known physics. He sends the damaged worker robot to destroy the arch, but barbed tendrils hurl it away.
With no technological solution, Nyr devises a desperate plan. His locator beacon, surgically embedded in his torso, allows the satellite to track him. If he removes it and places it at the arch, he can trigger the orbital strike to obliterate the gateway. At dawn, he asks Lyn to cut into him below his ribs. Horrified, she drives the sword in. Nyr rips out the beacon, drops it before the arch, and screams at the sky. Lyn hauls his body onto the reactivated worker robot, and as the machine lurches skyward, fire from heaven obliterates the arch behind them.
The demon's corruption begins disintegrating across the landscape. Esha, drawing on her knowledge of many languages, finds a word fundamental enough for the robot to understand, "home," and the machine carries Lyn and Nyr back to the tower. Lyn forces the outpost systems to produce a healing capsule and rolls his body into it. At her mother's court, no one believes Lyn until Esha arrives to corroborate the story. The queen is cold: Lannesite never cared about the Ordwood, and a weakened forest region might have been preferable.
Nineteen days later, a messenger arrives with an invitation from the tower. Nyr wakes to find the satellite no longer responds; with his beacon destroyed, he is presumed dead. He confirms the demon's corruption is fading. He decides he is no longer an anthropologist, his objectivity irrevocably compromised. The outpost is just a tower now. He is, functionally, a magician. He regrows Allwer's severed fingers, tells Lyn he wants to live among her people, and considers taking an apprentice. Lyn's eyes light up for a world where sorcerers and wonders still exist. Nyr lifts the DCS, and for a moment, he is happy.