12-year-old Ted Wiggins lives in Thneedville, a walled city where all vegetation and plant-life are artificial. Ted has a crush on environmentalist Audrey, and decides to impress her with a "real tree". His grandmother Norma tells him the legend of the Once-ler, who knows what happened to the trees. Leaving Thneedville in search of the Once-ler, Ted discovers that the outside world is a barren, contaminated wasteland. He finds the Once-ler, who agrees to tell him the story of the trees over multiple visits. The next time he leaves home, Ted encounters Thneedville's greedy mayor Aloysius O'Hare whose company sells bottled oxygen to the polluted city. Explaining that trees, and the oxygen they produce freely, pose a threat to his business, O'Hare pressures Ted to stay in town, but Ted continues to visit the Once-ler.
The Once-ler recounts how, as a young inventor, he arrived in a lush forest of animals and Truffula trees. Cutting down a tree, he was confronted by the Lorax, the guardian of the forest who "speaks for the trees". After attempting to force the Once-ler out, the Lorax convinced him not to harm any more trees. Using the Truffula tree, the Once-ler created the "Thneed", a piece of cloth with multiple uses, which became a major success. His unscrupulous relatives persuaded him to resume chopping down trees to mass-produce Thneeds, leading to enormous profits but deforestation and pollution. After harvesting the last Truffula tree, the Once-ler was ruined, abandoned by his family, and became a recluse. The Once-ler was left heartbroken and unbearably guilty for destroying the forest. With the region uninhabitable, the Lorax sent the animals away to find a new place to live, and vanished into the sky. The Lorax left a single word on a small pile of rocks: "Unless".
The Once-ler gives Ted the last Truffula seed in hopes he can regrow the forest and make others care about trees. Ted returns home to plant the seed, which is spotted by O'Hare's city-wide surveillance. Enlisting the help of Audrey and his family, Ted is pursued by O'Hare to the center of town. O'Hare rallies the citizens against Ted, telling them that trees are dangerous and filthy, but Ted uses an earthmover to knock down a section of the city wall, revealing the environmental destruction outside. Inspired by Ted's conviction, the crowd turns on O'Hare, and the seed is finally planted. Time passes and the land begins to recover; new trees sprout, animals return, and the now-elderly Once-ler reunites with the Lorax.
The film ends with a quote from Dr. Seuss: "Unless someone like you cares a whole awful lot, nothing is going to get better. It's not."
Danny DeVito as The Lorax, a mystical orange furry creature with a yellow moustache.
Ed Helms as The Once-ler.
Zac Efron as Ted Wiggins, an idealistic 12-year-old boy. He is named after the author of the book, Dr. Seuss (Theodor Geisel).
Taylor Swift as Audrey, Ted's love interest. She is named after Audrey Geisel, Dr. Seuss's wife.
Betty White as Grammy Norma, Ted's wise-cracking grandmother and Mrs. Wiggins's mother.
Rob Riggle as Aloysius O'Hare, the greedy mayor of Thneedville and head of the "O'Hare Air" company that supplies fresh air to Thneedville residents.
Jenny Slate as Mrs. Wiggins, Ted's neurotic mother and Grammy Norma's daughter.
Nasim Pedrad as Isabella, the Once-ler's mother.
Stephen Tobolowsky as Uncle Ubb, the Once-ler's uncle.
Elmarie Wendel as Aunt Grizelda, the Once-ler's aunt. This was Wendel's final film role before her death on July 21, 2018.
Danny Cooksey as Brett and Chet, the Once-ler's twin brothers.
Chris Renaud as assorted forest animals.
Additional voices are provided by Debi Derryberry.
also known as Dr. Seuss' The Lorax
based on Dr. Seuss' children's book of the same name