Ted Wiggins is an idealistic boy, who lives in “Thneedville”, a walled city that, aside from the human citizens, is completely artificial; everything is made of plastic, metal, or synthetics with no living plants. Ted has a crush on local environmentalist Audrey, who wants to see a “real tree” more than anything in the world, and decides to find one in order to impress her. His energetic Grammy Norma secretly tells Ted the legend of the Once-ler, who will tell anyone about trees if they brought him fifteen cents, a nail, and a shell of a great-great-great grandfather snail. When Ted leaves Thneedville in search of the Once-ler, he discovers that the outside world is a contaminated, empty, barren wasteland. Once the boy finds him, the Once-ler agrees to tell Ted about the trees on the condition that he listens to the story over multiple visits. Ted agrees, but on his way home, he encounters the mayor of Thneedville, Aloysius O’Hare, who is also the proprietor of a company that sells bottled oxygen to Thneedville residents. O’Hare explains to Ted that because trees produce oxygen free of charge, he considers it a threat to his business whenever he hears people talking about them. After revealing that he has “security camera eyes” all over the city, O’Hare pressures Ted to stay in town. However, Ted continues to sneak out of O’Hare’s sight (with his grandmother’s encouragement) and learns more of the trees’ history.
Over Ted’s various visits, the Once-ler recounts the story that when he was a young man, he departed his family to find good material for his Thneed invention and make a business. After stumbling upon a lush Truffula Tree forest valley, the Once-ler meets the guardian of the forest, the Lorax, after cutting down a Truffula Tree. The Lorax urges the Once-ler to leave the forest, but the Once-ler refuses. Eventually, the Once-ler promises not to chop another tree down, and the two seem to begin a friendship of sorts. Then, the young businessman’s Thneed invention becomes a major success and the Once-ler’s family arrives to participate in the business. At first keeping his promise, the Once-ler continues Thneed production by harvesting the Truffula Tree tufts in a slow, but sustainable manner. However, his greedy and lazy relatives soon convince him to resume chopping down the trees. Over time, the Once-ler’s deforestation spirals into a mass overproduction. Flushed with wealth, the Once-ler rationalizes his short-sighted needs into arrogant self-righteousness, and the Lorax’s helpless protests do not stop him. The Once-ler pollutes the sky, river, and landscape, until the very last Truffula Tree falls. With no further chance of business, he is left broken and abandoned by his family, with his mother disowning him, and with the region uninhabitable because of his business’s pollution, the Lorax sends the animals off to find a new place to live before he departs into the sky, leaving only a stone-cut word: “Unless”. Distraught and ruined, the Once-ler becomes a recluse.