As a young Boy stumbles upon alone red-and-white-striped hat, its curious owner, the Cat in the Hat introduces the boy to the wondrous world of Seuss ("Oh, The Thinks You Can Think!").
The Cat begins with the story of Horton the elephant who lives in the Jungle of Nool with Jungle Citizens, the Bird Girls, and several other unique creatures. One day, Horton is enjoying his bath when he hears a voice on a speck of dust asking for help ("Horton Hears A Who").
Horton delicately places the speck onto a clover just as Sour Kangaroo and Young Kangaroo bounce along. They inform Horton that there couldn't possibly be a person on that speck ("Biggest Blame Fool"). The Wickersham Brothers join in teasing. The Cat takes on the role of a talk show host and interviews two neighbor birds named Gertrude McFuzz and Mayzie La Bird about Horton hearing things that don't exist.
As the naysayers leave, Gertrude stays behind to marvel at the uniqueness of Horton ("Biggest Blame Fool Playoff/Gertrude McFuzz").
The cries of help return and this time, Mr. Mayor and Mrs. Mayor reveal themselves to Horton as citizens of a tiny place called Whoville. The boy from the beginning becomes Jojo, their son. And all of the Whos plead to Horton asking him to save their planet ("Here on Who").
Mr. Mayor and Mrs. Mayor share the joys and difficulties of parenting ("How To Raise A Child").
JoJo is blamed for the Cat's mischief. He tries to take his bath, but the Cat isn't done ("Oh, The Thinks You Can Think! - Reprise").
The Cat and JoJo are swept underwater with Fish all around ("It's Possible - Part 1")!
The tub overflows and Mr. and Mrs. Mayor blame JoJo, but JoJo knows deep down that anything is possible ("It's Possible - Part 2").
Meanwhile, Horton continues to guard the clover despite the other animals' judgments. Horton and JoJo bond in their imaginations ("Alone In The Universe").
The Cat wakes JoJo from his dream to tell him more stories ("The One Feather Tail Of Gertrude McFuzz"/"Amayzing Mayzie").
Gertrude wanted a more extravagant appearance, so she traveled to the doctor for help ("Amayzing Gertrude - Part 1").
She begins to take pills and her one-feather tail grows, but Gertrude can't stop there! She takes more pills, and her tail grows longer and longer ("Amayzing Gertrude - Part 2").
As Gertrude leaves, the Wickersham Brothers steal the clover away ("Monkey Around").
Horton chases them through great stretches of jungles, beaches, and sand, but the monkeys hand over the clover to Vlad Vladikoff, a black-bottomed eagle who drops the Whos into a field of clovers ("Chasing The Whos").
Horton searches frantically for the Whos as Gertrude reappears with a new fortified tail. She tries desperately to get Horton's attention, but Horton is focused on his search for the clover ("Notice Me, Horton").
Mayzie reappears and asks Horton if he would consider sitting on the egg in her nest ("How Lucky You Are").
Horton agrees ("Mayzie's Exit/Horton Sits On The Egg/Dilemma/ Hunters").
