One night, a little boy wanders down 'the street of the lifted Lorax.' Along the dark street, he comes to the residence of a creature named The Once-ler, a being who is never seen, except for his long green arms. He takes up an audience with the boy, and begins to explain the tale regarding the Lorax.
The land once thrived with truffula trees when the Once-ler first came to the area in a horse-drawn cart. Living among the foliage are the brown Bar-ba-Loots, who eat truffula fruit from the local trees. In the nearby pond live the Humming Fish, and overhead fly the Swomee Swans. The trees amaze the Once-ler with their texture and scent, and he soon builds a small shop in the area. After cutting down a truffula tree, the Lorax pops out of it's stump. The Lorax claims to speak for the trees, and demands to know what the Once-ler is doing. The Once-ler explains that he is using the truffula tree's tufts to make something called a Thneed...a fine thing that all people need.
The multi-purpose item soon attracts the attention of many customers, and the Once-ler continues to cut down more truffula trees. When it seems that the business is growing too big for just one Once-ler, he calls his relatives to help him with his thriving business.
The business keeps growing and growing, until finally the Lorax demands an audience with the Once-ler, explaining that with the number of trees having been cleared, the Bar-ba-Loots have run out of truffula fruits, and have left to find new sources of food.
The Once-ler ignores this, but is then met by the Lorax again, who explains that the smog from the factories has overwhelmed the Swomee Swans. Unable to sing because of the smog, the Lorax sends them off. The Once-ler claims he will 'think about it,' but this answer does not sit well with the Lorax. The Lorax then reveals that the factory's pollution is gumming up the Humming Fish's pond. With their home now so polluted that they can't hum, the fish are sent away as well.
The Once-ler it seems is about to listen to reason, until his secretary explains that the company's stock report has come in, and 'Thneeds, Inc' is doing exceptionally well. The Once-ler then scoffs at the Lorax, claiming that he will keep on making Thneeds, regardless of the consequences. As he finishes his taunt, the sound of an ax is heard, and the two are witness to the leveling of the last truffula tree. With no more trees, the factory shuts down, and the Once-ler's relatives pack up and leave.
Under the polluted skies, the empty factory sits as the Once-ler and the Lorax contemplate. Finally, the Lorax sadly glances at the Once-ler, before lifting himself by the seat of his pants, and disappearing through a hole in the smog. On the spot where the Lorax last stood, sits a small pile of rocks, with a word carved into one: "Unless."
The tale then switches back to the Once-ler talking to the boy, who is given a truffula seed by the Once-ler, encouraging the boy to help revitalize the long-dead trees, with the possibility that the Lorax and his friends may then come back.