"Part I: World Without End"
While working on his graduate studies in biology in the mountains, Ish is bitten by a rattlesnake. As he heals from the bite, he gets sick with a disease that looks like measles. He recovers and makes his way back to civilization, only to discover that most people died from the same disease. He goes to his home in Berkeley. As he travels, he observes the world in the light of ecology, watching it adapt to the loss of humans. In the city near his home Ish meets few human survivors — a man drinking himself to death, a couple who seem to have lost their sanity, and a teenage girl who flees from him as someone dangerous. He comes across a dog, friendly and eager to join him. The dog, now named Princess, swiftly adopts Ish as her new master and sticks by him for much of the book. Wondering if his observations are typical of humanity in general, he sets out on a cross country tour, traveling all the way to New York City and back, scavenging for food and fuel. As he travels, he finds small pockets of survivors, but he doubts that they will survive the loss of civilization.
He returns to his home in California, and finds a woman, Emma (Em), living nearby. They agree to consider themselves married and have children. They are joined by other survivors. Over time the electricity fails and the comforts of civilization recede. As the children grow, Ish tries to instill basic academics, teaching reading, arithmetic and geography.
"Quick Years"
During this period of time, Ish and Em meet many other people such as Ezra, George, and many others. This section goes all the way to the end of Year 21. The community, within this time period, started to call the years by events that happened in the Year. Many children were born in these years and within them was Joey, Ish's favorite son. Joey is Ish's favorite son because not only is Joey very similar to Ish, but Ish believes that Joey is the key to the future.
"Part II: The Year 22"
Chapters 1-5
Twenty-two years later, the community flourishes. The younger generation adapts easily to the more primitive world. They come to have a better grasp of the natural world than the adults, and when running water fails, the younger generation comes to the rescue, knowing where flowing streams may be found. The children see no need for structured academics and Ish isn't a natural teacher. Only one child, his son Joey, seems to be able to grasp and use academic skills. Ish increasingly sees Joey as the future leader and brains of the community.
Ish turns his attention from ecology to his newly forming society. One thing that he notices is that the children are becoming very superstitious. One day Ish asks for his hammer, an antique miner's tool found in the mountains, which he habitually carries around, and finds the children are afraid to touch it. It is a symbol for them of the old times. The long-dead Americans are now like gods—and Ish is too.
Ish becomes disturbed at his community's lack of ambition to learn and work. He tries to motivate them so often with speeches that the kids think this is simply his line, safe to be ignored. In an attempt to motivate them, Ish mentions the idea of a cross country exploration, and his son Robert and another boy Richard start out in a jeep.
Chapters 6-11
Robert and Richard return from their trip. They explored east across the country until they met impassable roads near Toledo, Ohio. They discovered several societies in their travels including a black family whom Ish had met on his cross-country journey years earlier. They brought back a man named Charlie, who gives Ish a bad feeling. Soon it is obvious that Charlie is after Evie, a girl the community regards as outside the acceptable gene pool—she has an adult body and the mind of a small child. Ish confronts Charlie and is intimidated; he feels alone and lost about what to do. Em takes control, calling a meeting of the adults. Ish isn't alone—they are a tribe. Under Em's insistence, the tribe's four adults vote on Charlie's fate. Em insists that they cannot wait until harm is done, that they have responsibility to protect their children. They unanimously vote to execute him.
The incident with Charlie makes Ish reflect that he is really not a nation builder, but he keeps trying. He begins practical lessons, such as planting corn. Then, typhoid fever erupts among them, perhaps carried by Charlie. Joey dies of typhoid, and this devastates Ish. With Joey gone, Ish decides teaching academic topics will be a fruitless effort. He worries what will become of his people when ammunition and matches are gone. He decides instead to teach his people to survive. He begins by inspiring the children to build bows and arrows.
"Quick Years"
The years flow by. Ish's lessons begin to take — and the community begins to grow corn and make and play with bows and arrows. Ish presides at meetings, his hammer a symbol of his status. He is given respect, but his ideas are ignored by the younger men. The Tribe merges with another nearby group. The "Americans" (those born before the Great Disaster) die off, until only Ezra and Ish are left, two old men. After Ezra dies, Ish becomes a sort of god, the last American, from whom the young men go to and demand answers.
"Part III: The Last American"
Ish spends most of his elderly life in a fog, unaware of the world. Superstition has set in; the tribe has reverted to a primitive lifestyle, hunting with dogs (the descendants of Ish's first dog) and bow and arrow. Occasionally the fog in his mind lifts. During one such time, he finds himself aware of his great-grandson Jack, who stands before him. Jack shows him that the bow and arrow have become more reliable than the gun, whose cartridges don't always work. The children of the world are taking the toys of their youth and improving them on their own. During his last lucid moments, Ish realizes that the former civilization is now totally gone. But he also wonders if the new world is that much worse off than the old world, and finds himself hoping that the new world will not rebuild civilization and its mistakes.