Sam Gribley is a 12-year-old boy who intensely dislikes living in his parents' cramped New York City apartment with his eight brothers and sisters. He decides to run away to his great-grandfather's abandoned farm in the Catskill Mountains to live in the wilderness. The novel begins in the middle of Sam's story, with Sam huddled in his treehouse home in the forest during a severe blizzard. Frightful, Sam's pet peregrine falcon, and The Baron, a weasel, share the home with him. In a flashback, Sam reminisces about how he came to be there.
Sam heard about his grandfather's abandoned farm near Delhi, New York, learned wilderness survival skills by reading a book at the New York City Public Library, and how Sam's father permitted him to go to Delhi, and, while he didn't think that Sam would survive, said that if he made it, to tell someone in town. Unable at first to locate the farm, Sam tries to survive on his own but finds his skills are not up to the task. He meets Bill, a man living in a cabin in the woods, who teaches him how to make a fire. Sam goes into town and is told where his grandfather's land is. Sam finds the farm but discovers the farmhouse is no longer standing.
A willow whistle, similar to the one made in the novel My Side of the Mountain.Sam forages for edible plants and traps animals for food. He uses fire to make the interior of the hollow tree bigger. Seeing a peregrine falcon hunting for prey, Sam decides he wants a falcon as a hunting bird. Sam goes to town and reads up on falconry at the local public library. He steals a chick from a falcon's nest and names the bird Frightful. Later, Sam hides in the woods for two days after a forest ranger, spotting the smoke from Sam's cooking fire, came to investigate.
In the fall, Sam makes a box trap to catch animals to eat, and catches a weasel. Sam calls the weasel The Baron for the regal way the animal moves about. When a poacher illegally kills a deer, Sam steals the carcass, smokes the meat, and tans the hides. Frightful proves very good at hunting. Sam prepares for winter by hunting, preserving wild grains and tubers, smoking fish and meat, and preparing storage spaces in hollowed-out trunks of trees. Finding another poached deer, Sam makes himself deerskin clothing to replace his worn-out clothes. Sam notices a raccoon digging for mussels in the creek and learns how to hunt for shellfish.
One day, Sam returns home and finds a man there. Believing the man is a criminal, he nicknames him "Bando" (an alternative version of "bandit"). The man is actually a professor of English literature and is lost. Bando spends 10 days with Sam building a raft, fishing, teaching him how to make jam, and showing him how to make a whistle out of a willow branch. Sam agrees to come to town at Christmas to visit Bando.
Sam makes a clay fireplace to keep his home warm. Sam steals two more dead deer from local hunters to make more clothes, begins rapidly storing as many fruits and nuts as he can, and builds his fireplace. Sam almost dies after he insulates his home too well, trapping carbon dioxide inside. Sick with carbon dioxide poisoning, Sam barely gets out alive. Sam returns to town just before Christmas. He meets Tom Sidler, a teenager who ridicules his appearance. Sam spends the night with Bando, who shows him the many newspaper articles about the "wild boy" living in the forest. Sam returns home and is surprised on Christmas Day by the arrival of his father. They are overjoyed to see one another again. Sam learns how animals behave in winter, even during blizzards. He overcomes a vitamin deficiency by eating the right foods.
In the spring, Matt Spell, a local teenager who wants to be a reporter, arrives at Sam's treehouse home. Sam doesn't want to be interviewed, but offers Matt a deal: Matt can come live with him for a week if Matt will not reveal his location. Matt agrees. A few weeks later, Bando visits Sam and they build a guest house. Matt spends a week with Sam, and at the end tells Sam he broke his promise. A short time later, Tom Sidler visits the farm and Sam realizes he is desperate for human companionship.
When Bando returns to check on Sam, Sam says he intends to return to New York City to visit his family. In June, Sam is surprised to find his family at the farm. His father announces that the family is moving to the farm. Sam is happy at first, then also upset because it means the end of his self-sufficiency. As the novel ends, Sam concludes that life is about balancing his desire to live off the land with his desire to be with the people he loves.
