A Friend of the Earth by T.C. Boyle looks back at the life of Tyrone O'Shaugnessy Tierwater, who is an elderly man when the novel opens. His involvement with an environmental fringe group changes his life in many ways that lead to his current situation at the beginning of the novel.

Tyrone O'Shaugnessy Tierwater is a middle-class man with a young daughter, Sierra. His wife dies when their daughter is only three. She is killed by a bee sting while they camp. Then he meets Andrea, who introduces him to Earth Forever!, which is an environmental group that uses extreme methods to achieve its ends.

Sierra, Ty, and Andrea work to save the forest in Oregon from loggers. As a result of involving Sierra in a dangerous situation, she's removed from Ty's custody. Ultimately he takes her back from the foster family and he, Andrea, and Sierra move into a cabin in California. After a while, the three are very unhappy with their isolated lifestyle. Ty decides to give himself up after he and Andrea are filmed living for a month off the forest alone—no clothes or other supplies included—to raise awareness. Ty turns himself in at the end and spends a year in prison while Andrea takes care of Sierra.

After he gets out, he's unable to keep himself from continuing reckless acts to support Earth First! and eventually is caught in the act. He has to serve four more years in jail since he breaks his parole. When he gets out of prison, Sierra is a young adult who is even more radical. She lives for three years in a redwood tree to keep it from being cut down. He chooses to live near her so they can talk on phones from the bottom to the top of the three. Ultimately, however, Sierra falls out of the tree and dies.

Ty lives in relative poverty and works at Maclovio Pulchris's home in Santa Barbara. This is where the novel opens. Ty is seventy-five and the estate has a lot of animals that are on the cusp of extinction because of the ruination of the planet. They don't have a lot of resources, but they take care of the animals, and it keeps Ty out of poverty. Even social security is bankrupt in America by 2025 when the novel opens. Andrea calls Ty, and she and April Wind, who wants to know about Sierra, come to the estate.

Eventually, however, one of the lions escapes and kills some of the people on the estate. Ty and Andrea take the last living Patagonian fox and escape along with a car full of supplies. They go to live in the woods with her.