A musical version of Horton Hears a Who! (2008) and the second Dr. Seuss musical
A dust speck is dislodged from its obscure place and sent adrift through the Jungle of Nool. At the same time, Horton the elephant, the jungle's eccentric 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 society of microscopic creatures are living on that speck, he places it atop a clover. Horton finds out the speck harbors the city of Whoville and its inhabitants, the Whos, led by Mayor Ned McDodd (Steve Carell). He has a wife, Sally (Amy Poehler), 96 daughters (whose names all begin with the letter H), and one teenage son named JoJo (Jesse McCartney). Despite being the oldest and next in line for the mayoral position, JoJo doesn't want to be mayor, and because he's so scared of disappointing his father, he never speaks. Once Horton begins carrying the speck with him, the city starts experiencing strange phenomena (earthquakes and changes in the weather), and the Mayor finds his attempts to caution Whoville challenged by the Town Council, led by the opportunistic yet condescending Chairman (Dan Fogler).
The Mayor finds out from Dr. LaRue (Isla Fisher) that Whoville 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. The head of the jungle, the Kangaroo (Carol Burnett), not believing Horton's beliefs, (and holding a personal grudge against him for always overshadowing her authority) attempts several times to demand that Horton give up the speck, but Horton does not and defies her. Also taking force toward Horton are the Wickersham Brothers, a group of bullying monkeys who love making misery. Eventually, the Kangaroo, upon learning Horton's defiance, enlists a vulture named Vlad Vladikoff (Will Arnett) to get rid of the speck by force.
Vlad manages to steal the clover away from Horton and drops it into a massive field of identical pink clovers, causing Whoville to fall into pieces. After unsuccessfully picking nearly 3,000,000 clovers, Horton eventually recovers the clover (exactly the 3,000,000th clover). In an instant, the Kangaroo finds out that Horton still has the speck, and decides to rally the jungle community into confronting Horton, saying that Horton's goal will lead to anarchy. Upon cornering him, the Kangaroo offers Horton to escape punishment by renouncing Whoville's existence. When Horton refuses, in spite of his heartfelt speech, she orders the animals to rope and cage him, and to have the speck and the Whos destroyed 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 may hear them, assisted by Jojo's "Symphonophone", an invention which creates a huge musical contribution, but still fails to penetrate the surface. As Horton lies beaten and captured, Kangaroo easily takes the clover, and slowly drops it towards the boiling oil. At the last minute, JoJo grabs the horn used to project Horton's voice, runs up the highest tower and yells "YOPP!", breaking through the sound barrier (and finally speaking for the first time in the film) just before the speck hits the oil.
Kangaroo's son, Rudy (Josh Flitter) grabs the clover and returns it to Horton, refusing his mother's orders to return to her pouch. The animals realize that Horton was right all along and shun the Kangaroo for tricking them. While being praised for his integrity by his neighbors, Horton even forgives the now-regretful Kangaroo, and she gratefully accepts his friendship with a makeshift umbrella for Whoville. Here, the people of Whoville and the animals of Nool gather in song and recite the chorus from "Can't Fight This Feeling". The film ends with the narrator revealing that the Jungle of Nool, or rather Earth, is just one speck, like Whoville, among numerous others, floating in outer space.
After the credits, the mayor is relaxing in his chair when he sees a small dust speck and hears a call for help coming from it, much to his dismay.
- Horton the Elephant, voiced by Jim Carrey, the main protagonist of the film.
- Ned McDodd, the Mayor of Whoville, voiced by Steve Carell, and the deuteragonist.
- Jane Sour Kangaroo is voiced by Carol Burnett, and one of the two main antagonist of the film..
- JoJo McDodd is voiced by Jesse McCartney, and the secondary tritagonist.
- Rudy is voiced by Josh Flitter.
- Morton the Mouse is voiced by Seth Rogen, and the tritagonist.
- The 96 Daughters of Ned McDodd are voiced by Selena Gomez.
- Sally O'Malley is voiced by Amy Poehler.
- Dr. Mary Lou Larue is voiced by Isla Fisher.
- Vlad Vladikoff is voiced by Will Arnett, one of the two main antagonist of the film.
- Yummo is voiced by Dan Fogler, and the secondary antagonist
- Mrs. Quilligan is voiced by Jaime Pressly.
- Jessica Quilligan is voiced by Laura Ortiz
- Tommy is voiced by Jonah Hill.
- Katie is voiced by Joey King.
- Miss Yelp, voiced by Niecy Nash
- The Chairman, voiced by Dan Fogler and the tertiary antagonist.
- The Wickersham Brothers, voiced by Frank Welker and Dan Castellaneta, and the secondary antagonists
- Charles Osgood as The Narrator