The novel begins on a wintry March afternoon in the isolated white farmhouse where the Murrys live. The one unusual room of the house is the laboratory where, as established in previous books, the Murry parents, both famous scientists, do research into polymers, virtual particles, and quasars. By messing around with their father's computer, Sandy and Dennys, the Murry twins, are transported to a hot, barren wilderness, where, almost immediately, they are badly burned by the blazing sun.

When the twins are rescued by a tent-dwelling family of small brown people who help them heal, they discover that, in this new world, humans live for hundreds of years and that several of them converse with their god, whom they call "El." It is some time before the twins learn that the "patriarch" of the family that takes them in is Noah and that the world will shortly be...