The World Tree rises up out of the seething clouds like a green mountain, lifting its children up to the light. All creation nestles in its gigantic branches: all take shelter beneath its canopy. There is no world besides this one -- or so the priests in Argos city would have everyone believe. What then if the green God should wither away, or withdraw Her blessings from her children?

Tymon is an orphaned boy growing up at Argos seminary, in the lush heart of the Central Canopy. The Argosian priests have declared science to be a heretical pursuit, and banned travel beyond the confines of the Tree. But Tymon yearns to discover new horizons. He longs to break free of the seminary. When he discovers an interloper in the city baths -- a foreigner, a female, one of the stigmatised Nurian pilgrims brought to the town every year as slaves -- his life changes forever.

Punished for his temerity and exiled to the dry and forsaken Eastern Canopy to serve out his indenture, he finds that there are different ways of interpreting the cosmos beyond those taught to him by the priests in Argos. He discovers that the heresy of Grafting, belief in the mystic 'Tree of Being', still persists in the eastern colonies. And he meets Samiha, the girl who holds the key to his own latent powers, as well as the ultimate fate of his world.