The book concerns an elephant named Horton, who is convinced by an irresponsible bird named Mayzie to sit on her egg while she takes a short "break", which proves to last for months. Naturally, the absurd sight of an elephant sitting atop a tree makes quite a scene. Horton is laughed at by his jungle friends, exposed to the elements, captured by hunters, forced to endure a terrible sea voyage, and finally placed in a traveling circus. However, he refuses to leave the nest through all of these, because he promised Mayzie he would look after the egg ("I meant what I said and I said what I meant, And an elephant's faithful, one hundred per cent!") Mayzie returns to the circus once the egg is due to hatch, and demands its return without offering any reward for Horton. However, when the egg hatches, the creature that emerges is a cross between Horton and Mayzie (an "elephant-bird"), and Horton and the baby are returned happily to the jungle, rewarding Horton for his persistence.