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, chops one down and uses its silk-like foliage to knit a Thneed, an impossibly versatile garment. The Lorax, who "speaks for the trees," emerges from the stump of the Truffula and voices his disapproval both of the sacrifice of the tree and of the Thneed itself. However, the first other person to happen by purchases the Thneed for $3.98, so the Once-ler is encouraged and starts a business making and selling Thneeds.
The Once-ler's small shop soon grows into a factory. The Once-ler's relatives all come to work for him and new vehicles and equipment are brought in to log the Truffula forest and ship out Thneeds. The Lorax appears again to report that the small bear-like Bar-ba-loots, who eat Truffula fruits, are short of food and must be sent away to find more. The Lorax later returns to complain that the factory has polluted the air and the water, forcing the Swomee-Swans and Humming-Fish to migrate as well. The Once-ler is unrepentant and defiantly tells the Lorax that he will keep on "biggering" his business, but at that very moment, one of his machines chops down the very last Truffula tree.
Without raw materials, the factory shuts down and the Once-ler's relatives promptly abandon him in the now-decimated environment. The Lorax says nothing but with one sad backward glance lifts himself into the air "by the seat of his pants" and disappears behind the smoggy clouds. Where he last stood is a small pile of rocks with a single word: "UNLESS". The Once-ler ponders the message for years, in solitude and self-imposed exile.
In the present, as his buildings fall apart around him, the Once-ler at last realizes out loud what the Lorax meant: "Unless someone like you cares a whole awful lot, nothing is going to get better. It's not." He then gives the boy the last Truffula seed and urges him to grow a forest from it, saying that, if the trees can be protected from logging, then the Lorax and all of his friends may come back.