A Nomadic family in the future is allied with an octopus-like alien who can survive in earth's new atmosphere, which has combined with nitrogen. Humans must live in shelters with oxygen-generating plants, or use suitable breathing equipment. Some of earth's original life forms have mutated to survive in the changed atmosphere. Since almost no metals can exist in the corrosive atmosphere, any technology is based on ceramics or glass.
Some humans are suspicious of the aliens, blaming them for the change in the atmosphere since they are adapted to it. The family has an almost fatal encounter with a group of such people, who are holding another alien hostage. However the two aliens are able to pool memories biochemically so that they become the same personality in two bodies. Their combined knowledge and skills help the family to escape. The journey reveals just who the aliens are, where they came from, and why the earth's atmosphere changed.