The Lorax opens as a young boy walks down away from a town on the Street of the Lifted Lorax on a polluted, empty hill. He arrives at the tall, winding house of the Once-ler, and pays him in exchange for his story.
The Once-ler proceeds to tell his story. It begins with the Once-ler's arrival at a flourishing forest of Truffula trees and many interesting animals. This valley was pure and clean with life teeming throughout it. The Once-ler is captivated by the soft and beautiful tufts of the Truffula trees, and decides to chop down a tree and build a garment called a Thneed out of its tuft.
Immediately after this first chop, we are introduced to the Lorax, a small creature who appears to advocate for the trees that the Once-ler has begun chopping down. He objects to the chopping down of the Truffula trees, but the Once-ler pays him no mind, and immediately begins expansion of his Thneed business. He employs all his relatives, and chops down more and more Truffula trees.
The Lorax arrives at the Once-ler's door with various animals in tow, forcing him to see the effects of his factory's destruction of environment and pollution on the wildlife. One by one, he sends the Brown Bar-ba-loots, the Swomee Swans, and the Humming Fish away to seek a better life. Each time, the Once-ler is filled with momentary remorse, but then continues to expand his business.
Finally, the Once-ler chops down the last Truffula tree. His business shuts down and he sends his relatives away. The Lorax himself departs into the smoggy clouds, leaving only the word "UNLESS" engraved on a pile of stones.
From that point on, the Once-ler secluded himself away in his tower, reflecting on the lessons of the Lorax—until this young listener arrived. He passes down the last Truffula Tree seed to the boy, and tells him that it is his responsibility now to make things better and plant back the forest. Only then, he says, will the Lorax and his friends return.