A young unnamed boy living in a polluted area visits a strange isolated man called the Once-ler on the Street of the Lifted Lorax. The boy pays the Once-ler fifteen cents, a nail, and the shell of a great-great-great grandfather snail to hear the legend of how the Lorax was lifted and taken away.

The Once-ler tells the boy of his arrival in a beautiful valley containing a forest of Truffula trees and a range of animals. The Once-ler, having long searched for such a tree as the Truffula, cut one down and used its silk-like foliage to knit a Thneed, an impossibly versatile garment. The Lorax, who "speaks for the trees," emerged from the stump of the Truffula and voiced his disapproval both of the sacrifice of the tree and of the Thneed itself. However, the first other person to pass by purchased the Thneed for $3.98 (equivalent to $25 in 2019), so the Once-ler was encouraged and started a business making and selling Thneeds.

The Once-ler's small shop soon grew into a factory. The Once-ler's relatives all came to work for him and new vehicles and equipment were brought in to log the Truffula forest and ship out Thneeds. The Lorax appeared again to report that the small bear-like Bar-ba-loots, who eat Truffula fruits, were short of food and had been sent away to find more. The Lorax later returned to complain that the factory had polluted the air and the water, forcing the Swomee-Swans and Humming-Fish to migrate as well. The Once-ler was unrepentant and defiantly told the Lorax that he would keep on "biggering" his business, but at that very moment, one of his machines chopped down the very last Truffula tree of them all.

Without any raw materials, the factory shut down and the Once-ler's relatives promptly abandoned him in the now-decimated environment. The Lorax said nothing but with one sad backward glance lifted himself into the air "by the seat of his pants" and disappeared behind the smoggy clouds. Where he last stood is a small pile of rocks with a single word: "UNLESS". The Once-ler pondered the message for years, in solitude and self-imposed exile.