The super-short plot basically goes like this:
Tally Youngblood wants to have the surgery that will make her pretty; but then she gets caught up in a rebellion against society and comes to appreciate a world where not everyone is pretty—and then (irony alert) she has to turn herself in to be turned pretty.
The slightly-longer-but-still-short plot summary breaks down nicely into three parts (thanks to the fact that the book has three parts), in a classic three-act structure:
Part I: The hero is introduced and accepts the adventure
Tally Youngblood is about to get the pretty surgery—which will be great, because her best friend already lives in the pretty part of the city. But when she almost gets in trouble (breaking into New Pretty Town), she meets Shay, an ugly who is a little more rebellious. Shay and Tally argue (and argue and argue) about whether the pretty surgery is a good thing. Eventually, Shay tells Tally about a hidden town in the wilderness where no one is pretty, but Tally doesn't want to go.
Unfortunately for her, she's been watched by Dr. Cable of the secret police (Special Circumstances), and Dr. Cable blackmails Tally into going to find this hidden town: if she doesn't help, Cable will never let Tally become a pretty.
Part II: The hero switches her allegiance, and then…
Tally goes off to find the hidden town, called the Smoke. It takes her a while, but she's very glad to find Shay safe and sound. And she's even happier to meet David, a boy who was born in the wilderness and so has different ideas about what's pretty. Different enough that he kind of falls for Tally.
Tally considers calling Dr. Cable, but she begins to like the Smoke. David also introduces Tally to his parents who tell her that the pretty surgery includes a side order of brain damage to make people manageable. So now Tally is definitely not going to call Dr. Cable and she even destroys the tracker that she's supposed to use to call Dr. Cable. But…
Part III: …the hero suffers a crushing defeat, but fights back and wins (partly, leaving open room for a sequel)
Now that Tally is happy in the Smoke, Special Circumstances comes and arrests everyone. Obviously. (Apparently the tracker that Tally had is activated by the damage of being thrown in the fire, just like your cellphone. Note: Do not throw your cellphone in fire.) Tally and David escape and go to the city to rescue the captives there, including David's mom and dad.
But they're too late: David's dad is dead and Shay has already been turned pretty. (The horror! The horror!) David's mom has come up with a way to cure the brain damage, but she needs a willing volunteer. Tally feels so guilty that she volunteers. The book ends with Tally confessing to David how she's responsible and giving herself up to the city authorities.