Beedie is a Bridger, one of the caste that uses the giant roots of the cavern wall to build the bridge towns that span the chasm. When the Elders of Topbridge decide they need to expand, Beedie is sent to measure a main root for cutting on one side of the chasm. Byle, one of the Bander family, is sent to do the other side. When the Banders make an issue of checking her measure, she asks that Byle's is checked too, and his is found short.
Instead of a whole group of Bridgers going to Beedie's root the following day, they go to Byle's to watch his technique. When she reaches the root, she finds that it is burning, and greenroot smoke is deadly poisonous. She quickly saws through the root herself, dangling half-dead in the smoke, and sees a strange creature flying toward her.
Beedie is rescued by Mavin Manyshaped, who can shift into any shape she wishes. With Beedie's help, she pretends to be from another Bridegtown, and Mavin finds that there is another Bird-Girl in Topbridge. The Girl had arrived some time ago and is kept by the Birders, the religious caste who believe that birds are the messengers of the Boundless.
Mavin visits the Birg-Girl and recognises her as her lost sister, Handbright, and also realises that Handbright is pregnant. The Banders are stirring up revolution against the Birders, who act as judges, and Beedie knows they are responsible for the fire on the root, which should have taken out most of the Bridger Elders. If there is no one older than him, Slysaw Bander would become chasm Elder, leader of the people.
Mavin and Beedie speak to the Elder Rootweaver and they decide to go on an adventure. They will go to the bottom of the chasm. The story put out will be that they seek treasure, but the truth is that something is destroying to the roots from the bottom, and if they don't find out what, the Bridges are doomed.
Along with Beedie and Mavin go Roges the Maintainer, and Mercald the Birder, who is the father of Handbright's child. As predicted, the Banders follow to try and take the unnamed treasure. On the way down, they encounter a huge slug, crawling and chewing it's way up one of the roots, but they still don't know why it has only arrived now.
Finally they make it down to the bottom, and find that answers to many mysteries. Years earlier, there was a bridge near the bottom called Waterlight, but it vanished one night, never to be seen again. Mavin is deeply suspicious when they meet a stranger who claims to be Mertylon, the leader of Waterlight from hundreds of years ago.
Soon they find out the truth, there are creatures living in the bottom which call themselves stickies, they eat by absorbing things that touch them and they found Waterlight and ate the people living there. When they ate the people, they absorbed their knowledge, and they still live down there, with the memories of the lost people.
The stickies explain that the giant slugs have arrived because they were cut off from their usual habitat by a rockslide downchasm, and have come up. While they try to think of a way to fix the problem, the Banders are still following. Mavin and Beedie come up with a plan, with the help of the stickies they trap the Banders above a pit of boiling water and drop them in.
The stickies agree to band together and destroy the giant slugs the same way and the group return to Topbridge. Handbright dies during childbirth, bearing twin boys, which Mavin decides to take back to the land of the True Game.