In a walled city devoid of nature, a young boy named Ted lives among artificial surroundings. Ted falls for a young girl named Audrey, who dreams of seeing a real tree. In an effort to win her affection, Ted sets out to find a way to obtain a tree, leading him to uncover the story of the Once-ler, a mysterious recluse who lives beyond the city outskirts. The Once-ler tells Ted about how he discovered a lush valley populated by Truffula trees and various wildlife, including the Lorax, a small, mustached guardian of the forest who speaks for the trees and protects the valley.

As the Once-ler tells his tale, he reveals how he started a booming business by chopping down Truffula trees to create a versatile garment called a Thneed. Ignoring the Lorax's warnings, the Once-ler's actions led to environmental degradation and the eventual disappearance of the Truffula trees. The story illustrates the consequences of environmental neglect and corporate greed. Ted learns the value of nature and becomes determined to make things right, but the task won't be easy, especially with the city's influential businessman trying to maintain control over the artificial environment.