Dr. Seuss' Horton hears a who

Jim carrey as HORTON and Steve Carrel as Ned the mayor of whoville

The film opens in the Jungle of Nool, where a drop of water falls off a leaf, causing a chain reaction. The small flower is crushed by a bur and a tiny dust speck is pushed off the flower and into the jungle. Meanwhile, a caring, imaginative elephant named Horton (Jim Carrey), the jungle's nature teacher, takes a dip in the pool. The dust speck floats past him in the air, and he hears a tiny yelp coming from it. Believing that an entire family of microscopic creatures are living on that speck, he places it on top of a pink clover that he holds in his trunk. Horton finds out the speck harbors the city of Who-ville and its inhabitants, led by Mayor Ned McDodd (Steve Carell). He has a wife, Sally (Amy Poehler), 96 daughters (Selena Gomez) who all have names that start with the letter H, and one teenage son named JoJo (Jesse McCartney). Despite being next in line for the mayoral position, JoJo refuses to become mayor of the city.

The Mayor finds out from Dr. LaRue (Isla Fisher) that Who-ville will be destroyed if Horton does not find a "safer more stable home." Horton resolves to place the speck atop Mt. Nool, the safest place in the jungle. This outlook earns Horton nothing but ridicule from the inhabitants of Nool, especially from the leader of the jungle, the Sour Kangaroo (Carol Burnett). The Kangaroo tries to get Horton to give up the speck, so as not to put supposedly ridiculous ideas into the heads of the children. Ever faithful to his two mottos, "A person's a person, no matter how small," and "I meant what I said, and I said what I meant, an Elephant's faithful 100%" (a reference to his other story Horton Hatches the Egg), Horton refuses. Also taking force toward Horton are the Wickersham brothers (Frank Welker and Dan Castellaneta), a group of bullying monkeys who love making misery.

The other Whos become suspicious of the various phenomena in their world (actually caused by Horton's various mishaps), the Mayor finally reveals the truth, but at first, the Whos do not believe him any more than the animals believe Horton. The Kangaroo enlists a buzzard named Vlad Vladikoff (Will Arnett) to get rid of the speck by force. Vlad manages to steal the clover with the speck on it, flee from a chasing Horton and drop it from hundreds of feet into a valley full of nearly identical clovers. The impact destroys Who-ville. Horton picks clovers through the field and finally finds it "on the 3 millionth flower." The city's destruction, combined with hearing Horton's voice through the drain pipe, is enough to convince the rest of the Whos that the mayor is not crazy, and they all tell Horton they believe in him.

The Kangaroo finds out that Horton still has the speck, and decides to rally the jungle community into confronting Horton. The Kangaroo offers Horton to give up the speck. Unable to convince Horton, she orders the animals to rope and cage him, and that the speck be burned in a pot of boiling "beezlenut" oil.

The Mayor enlists all of his people to make noise by shouting, "We are here," as well as playing a variety of instruments, so the animals can hear them. Horton is caged, and the Kangaroo takes the clover and drops it into the oil. At the last minute, JoJo grabs the horn used to project Horton's voice, runs up the highest tower and yells "YOP!" Just before the speck hits the oil, Jojo's sound causes their noise to be heard.

Hearing the Whos' cries, Rudy (Josh Flitter), the Kangaroo's son, grabs the clover and returns it to Horton, refusing his mother's orders to return to her pouch. The animals realize their mistake. While being praised for his integrity by his neighbors, Horton forgives the Kangaroo and she accepts his friendship. She and the animals resolve to join Horton in returning the clover to Mount Nool. Here the people of Who-ville and the animals of Nool gather in song and recite the chorus from "Can't Fight This Feeling", and begin walking the final stretch to the mountain.