The book tells the story of Horton the Elephant who, 
on the 15th of May in the Jungle of Nool, 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", 
inhabited by microscopic-sized inhabitants known as Whos.

It begins:

On the fifteenth of May, in the Jungle of Nool,
In the heat of the day, in the cool of the pool,
He was splashing…enjoying the jungle’s great joys...
When Horton the elephant heard a small noise.