The story begins with the background of Steven leaving to serve in the second world war, leaving his home near Ryhope Wood, and his distant, uncaring father. His recovery from wounds after the war in France and finally his reluctant return to his home. While he has been away his father has died and his brother Christian has married a girl called Guiwenneth.

When Steven arrives though, Christian is alone, Guiwenneth has gone, and he won't discuss it. Steven is disturbed by the changes in Christian, who is coming to resemble their obsessed, driven father, Huxley.

Shortly afterwards, Christian leaves abruptly in the middle of the night, going into Ryhope Wood. The wood has only a six mile perimeter, yet apparently Christian, like Huxley, can disappear inside for days or weeks.

While Christian is away, Steven has a strange encounter. A man, speaking another language, with arrows and primitive weapons, arrives at the farm wanting food. His dog begins to unearth something in the yard before being dragged away. When Steven investigates, he finds a girl's body, with an arrow protruding from the skull.

Upon Christian's return, Steven learns that it is Guiwenneth, yet Christian believes he can find her again. Huxley's mysterious work involved the wood, and the phenomena he observed there. The energy of the forest calls upon buried memories to create myth forms, mythagos. They are the figures of legend which emerge in times of great cultural danger, Robin Hood, Kind Arthur, other more primitive types.

Living close to the wood causes the wood to draw these myth forms from a person's mind and make them real. Huxley believed that there was an original myth-form, the Urscumug, and searched for it. But what he found was Guiwenneth. The warrior maid who fought against the Roman invaders, with her own myth cycle.

Huxley became hopelessly obsessed by her, but she loved Christian, and in Huxley's final days he tried to kill his son over her. The real problem was that Huxley could not truly believe in a woman who was violent, so she was too vulnerable, and died after being attacked in the woods. Christian believed that she would live again, created by his own mind interacting with the woods, and he searches for her.

When Christian again leaves, Steven spends some time reading his father's journal, learning about the wood. The times when his father had disappeared for up to a week in there, the subjective time had been several weeks, as time was distorted. The wood protects itself, turning explorers back on themselves, and not allowing them into the stronger and more vigourous zones.

Christian no longer returns to the house, and months go by before Steven tries to fly over the wood with the help of Keeton, an ex-army pilot doing local surveys. They nearly crash as the wood again rigourously protects itself.

Steven returns home to find that he has had a visitor. As the days pass, the visitor becomes more bold, and it is Guiwenneth. He too falls in love with her, and she with him, learning english and living with him. But it is not destined to last.

One night, when Keeton is visiting, Christian returns from the wood. Fifteen years older, with a wild following of barbarian soldiers, he almost kills Steven and kidnaps Guiwenneth. He says that he no longer cares whether she loves him or not, he only wants to have her and use her.

Just before he kills Steven, he is driven away by the arrival of the Urscumug, which hunts him. Created from their father's mind, filled with hatred for them, it has hunted Christian for all these years. A giant creature, twice the height of a man, a boar which stands on two legs and paints its face with the features of a man, the features of Huxley.

Steven and Keeton decide to enter the wood and rescue Guiwenneth, although Keeton has his own hidden reasons. Burned across the face during the war, he was wounded in a wood like Ryhope in France, which he has never managed to find again. He believes that in Ryhope, he can be healed.

As they journey through the wood, encountering mythagos, they learn that Christian is known as "the Outsider", and that he can only be killed by his kin. Keeton is increasingly disturbed by the idea that Steven has become a myth for the mythscape, but Steven accepts it and draws courage from the implied promise of success.

Keeton eventually leaves him to find his own way, and Steven makes his way to a valley in the myth of Guiwenneth, the valley which breathes, where her father is buried. At the end of the valley is a wall of fire, which the mythagos say guards the land of Lavondyss, where the souls of men are not tied to seasons.

Steven waits there and Christian arrives alone, claiming that Guiwenneth escaped him. Christian wants to enter Lavondyss, he believes that in there, the last fifteen years, and what he has become because of them, can be erased. That he can return to England and his life. He no longer cares about Guiwenneth one way or another.

Steven cannot make himself kill Christian, and Christian begins the ritual which will allow him to cross the fire, throwing dirt across the flames. But Steven remembers that he carries a talisman belonging to Christian and throws it to him while he crosses the flame. It strikes Christian in the face, knocking him down so that the fire consumes him.

When Guiwenneth finally comes to the valley, she is dying, Christian has stabbed her. While Steven holds her, the Urscumug comes to them, and Steven believes they will both die. But the creature takes her tenderly in its arms, and takes Steven's hand.

They walk to the end of the valley, and the Urscumug indicates that Steven must stay there, and Guiwenneth will cross the flames. But it also promises that she will return.

The final story is that the valley that breathes is named 'ritha muireog', 'where the hunter waits', but that in later times it was known as 'imarn uklyss', 'where the girl came back through the fire'.