In the novel, Earth now has one side constantly facing the sun (which is larger and hotter than it is at present) so it has become a veritable hothouse, where plants have filled almost all ecological niches.

Set in a far future, the Earth has locked rotation with the Sun, and is attached to the now-more-distant Moon, which resides at a Trojan point, with cobwebs spun by enormous spider-like plants.

The Sun is described as "paralysing half the heaven" and "always fixed and still at one point in the sky" and, with the increased light and heat, the plants are engaged in a constant frenzy of growth and decay, like a tropical forest enhanced a thousandfold.

The plants – many now omnivores – have filled all the ecological niches on the land and in the air, many evolving primitive nervous systems and, in some cases, eyes; of the animals in the forest only the descendants of four species of social insects remain – tigerflies (evolved from wasps), tree-bees, plant-ants and termights (from termites) – along with small groups of humans (a fifth of the size they are now); all other land and air animals have been driven to extinction by the vegetable kingdom, apart from a few shore dwellers.

The humans live on the edge of extinction, within the canopy layer of a giant banyan tree that covers the continent on the day side of the Earth.

Lily-yo, leader of a small, matriarchal human tribe, decides that the group should break up, as the adults are too old, and should go to the "Tips", the dangerous top levels of the forest, to go "Up".

Walking back to the forest through "Nomansland" – the lethal interface-area between land and sea – Gren is waylaid by a "morel", a sentient fungus which attaches itself to his head and forms a symbiotic relationship.

The morel explains to them that the world is about to end as the Sun brightens, and the strange, green columns they begin to see beaming into space are shafts of organic material, transferring to new star systems.

They board a traverser which is going to lift off to the stars (after being taken over by the morel which has now divided) – all except Gren, Yattmur, and the baby, who decide to return to the familiar forest – for the end of the world, while soon, will not occur within any of their lifetimes.