In this story, Horton discovers there is a microscopic community of intelligent beings called the Who's living on a plant that only he can hear. Recognising the dangers they face, he resolves to keep them safe. However, the other animals around him think Horton has gone crazy thinking that there are such beings.

After receiving an exotic small animal as a Christmas gift, a young man inadvertently breaks three important rules concerning his new pet, which unleashes a horde of malevolently mischievous creatures on a small town.