A few thousand years in the future, post-apocalypse rural Labrador has become a warmer and more hospitable place than it is at present. The inhabitants of Labrador have vague historical recollections of the "Old People", a technologically advanced civilisation which existed long ago and which they believe was destroyed when God sent "Tribulation" to the world to punish their forebears' sins. The society that has survived in Labrador is loosely reminiscent of the American frontier during the 18th century, with the technological status echoing the Amish of the United States.
The inhabitants practise a form of fundamentalist Christianity with post-apocalyptic prohibitions. They believe that to follow God's word and prevent another Tribulation, they need to preserve absolute normality among the surviving humans, plants and animals so follow a eugenics policy. Genetic invariance has been elevated to the highest religious principle, and humans with even minor mutations are considered "Blasphemies" and the handiwork of the Devil.
Individuals not conforming to a strict physical norm are either killed or sterilised and banished to the Fringes, a lawless and untamed area still rife with animal and plant mutations. Arguments occur over the keeping of a tailless cat or the possession of over sized horses. These are deemed by the government to be legitimate breeds either pre-existing or achieved through conventional breeding. The government's position is considered both cynical and heretical by many of the orthodox frontier community.
The inland rural settlement of Waknuk is a frontier farming community, populated with hardy and generally pious individuals. Ten-year-old David Strorm, the son of Waknuk's zealous and religious patriarch, has inexplicably vivid dreams of brightly lit cities and horseless carts that are at odds with his pre-industrial experience. Despite David's rigorous religious training, he befriends Sophie, a girl carefully concealing the fact that she has six toes on each foot. With the nonchalance of childhood, David keeps her secret. The subsequent discovery of Sophie's mutation and her family's attempted flight from the feared reprisals causes David to wonder at the brutal persecution of human "Blasphemies" and the ritual culling of animal and plant "Deviations". David and a few others of his generation harbour their own invisible mutation: they have telepathic abilities. David begins to question why all who are different must be banished or killed.
As they mature, David and his fellow telepaths realise that their unusual mutation would be considered a "blasphemy" and they carefully conceal their abilities. That their mutation cannot be directly detected allows their unusual abilities to remain undiscovered for a time. Eventually, some of the group are exposed and David, his half-cousin Rosalind, and younger sister Petra flee to the Fringes. They are quickly pursued by a group of villagers who intend to capture and interrogate them, however the group includes Michael, a fellow telepath who covertly assists their escape. Through the extremely strong telepathic abilities of Petra, they make contact with a more advanced society in distant "Sealand", intimated as being New Zealand. With Michael's help, David, Rosalind, and Petra elude their would-be captors and are rescued by a Sealand expedition sent to discover the source of Petra's telepathic transmissions. Due to limited fuel on the Sealand craft, they are unable to return to Waknuk to collect Rachel, so Michael remains behind to be with her.
Though the nature of "Tribulation" is not explicitly stated, it is implied that it was a nuclear holocaust, both by the mutations, and by the stories of sailors who report blackened, glassy wastes to the south-west where the remains of faintly glowing cities can be seen (presumably the east coast of the US). Sailors venturing too close to these ruins experience symptoms consistent with radiation sickness. A woman from Sealand, a character with evident knowledge of the Old People's technology, mentions "the power of gods in the hands of children".