The story opens in a desolate part of town - - the "wind smells slow-and-sour" and "no birds ever sing." A creepy character called the Onceler lives in a ramshackle building in the area and, in exchange for a few pennies, will explain what happened to the area and specifically, "how the Lorax was lifted away." That point is emphasized four times in the first few introductory pages:
-"somebody lifted the Lorax away"
-"why was it lifted and taken somewhere"
-"the Lorax was lifted away"
-"the Lorax got lifted and taken away"
The Onceler explains to a boy that he discovered the area a "long, long time back" and that, at that time, the grass was green, the sky and water were clean, and everywhere were beautiful trees and happy bears, fish, and birds. The illustrations in the book, which previously looked like this:

Now look like this:

But if this is supposed to be paradise, it's not my idea of paradise. There are no buildings or any other sign of modern society. And there's especially no people.
Into this Eden comes the Onceler, who immediately recognizes that the trees could be very valuable. He quickly builds a small workshop and turns parts of a "Truffula Tree" into a beautiful and valuable fabric. The Lorax makes his first appearance:
He was shortish. And oldish.
And brownish. And mossy.
And he spoke with a voice
that was sharpish and bossy.

Not exactly complimentary. The Lorax claims that he "speaks for the trees" and screams at the Onceler, demanding an explanation for his behavior. The Onceler shows the beautiful and useful fabric that he's created, but the Lorax dismisses the product as useless. He also insults the Onceler as "crazy with greed." To the Lorax's surprise, a customer immediately appears and buys the fabric. But the Lorax isn't chagrined. Rather, he simply attempts to resume berating the Onceler, who is no longer interested in listening.
In the following pages, the Onceler creates a business churning out fabric that is so successful, he's able to build an ever-expanding factory and employ his entire extended family. The Lorax eventually returns. He's (1) completely unmoved by the Onceler's success; and (2) made no effort to alter his behavior. During this visit, the Lorax angrily confronts the Onceler and tells him that he's cut down so many trees that the bears in the area no longer have anything to eat. Here, something very strange happens. The Lorax tells the Onceler that since the bears are having trouble finding food, he's "sending" them away. The Lorax "hopes" they'll find food elsewhere. In other words, the Lorax has no intention of helping the bears find food.
Between the Lorax's grating personality, the fact that even the Lorax has no plans of actually helping the bears, and the Onceler's tremendous business success, the Onceler dismisses the Lorax and continues to grow his business.
Eventually, the Lorax returns to confront the Onceler. This confrontation is extremely similar to the last. The Lorax yells at the Onceler, this time telling him that his factory has so polluted the air and water that the Lorax is "sending" the fish and birds "off." Once again, the Lorax makes clear he doesn't plan on actually helping the animals:
Where will they go?...
I don't hopefully know.
At this point, the Onceler is so angered by the Lorax's behavior that he announces he will do the exact opposite of what the Lorax has asked:
I yelled at the Lorax, "Now listen here, Dad!
All you do is yap-yap and say, 'Bad! Bad! Bad! Bad!
Well, I have my rights, sir, and I'm telling you
I intend to go on doing just what I do!
And, for your information, you Lorax, I'm figgering
on biggering
and BIGGERING
and BIGGERING
and BIGGERING,
turning MORE Truffula Trees into Thneeds
which everyone, EVERYONE, EVERYONE needs!"
The only question is whether the Lorax is the worst persuader ever or the worst persuader ever. And sure enough, as soon as the Onceler finishes his speech, we learn that his business has managed to cut down every single tree the business depended on. Without trees, the business folds and the jobless employees drive off in search of a new paycheck.
So, we now have a chastened Onceler and the Lorax, who supposedly deeply cares about the environment. Surely they'll work together now to try to fix things. Right? Nope, the Lorax flies away to an unnamed destination. Hold on, we were told at the beginning that "somebody lifted the Lorax away." That's not true at all. He saw a tough problem, realized all he was good at was irritating people, and lifted himself away.

All in all, the Lorax is possibly the most beloved failure in kid lit. An excellent use for this book would be to read it in class and then ask students to come up with ways the Lorax could have actually persuaded the Onceler.
The final touch is that the Onceler ends up the hero of the story. He had the foresight to save a Truffula Tree seed, and in the book's conclusion, gives it to a boy in the hopes the boy will grow new trees. That conclusion does a nice job of promoting the value of private ownership of property. But that ground has already been explained elsewhere.