It tells the story of Horton the Elephant who one day hears a small speck of dust talking to him. It turns out the speck of dust is actually a tiny planet, home to a city called "Who-ville." The Whos ask Horton to save their home from destruction to which Horton happily obliges, but in doing so is denounced by the other animals in the jungle for believing in something that cannot be seen.
Throughout the book Horton repeats the line "A person's a person, no matter how small."