Tally lives in a world where your sixteenth birthday brings aesthetic perfection: an operation which erases all your flaws, transforming you from an 'Ugly' into a 'Pretty'. She is on the eve of this important event, and cannot wait for her life to change. As well as guaranteeing supermodel looks, life as a Pretty seems to revolve around having a good time.

But then she meets Shay, who is also fifteen - but with a very different outlook on life. Shay isn't sure she wants to be Pretty and plans to escape to a community in the forest - the Rusty Ruins - where Uglies go to escape ' turning'. Tally won't be persuaded to join her, as this would involve sacrificing everything she's ever wanted for a lot of uncertainty. When she is taken in for questioning on her birthday, however, Tally gets sent to the Ruins anyway - against her will.

The authorities offer Tally the worst choice she could ever imagine: find her friend Shay and turn her in, or never turn Pretty at all. What she discovers in the Ruins reveals that there is nothing 'pretty' about the transformations...And the choice Tally makes will change her world forever.