(some minor spoilers from the first book follow)
Tally is pretty - something that happens to all sixteen-year-olds. Being pretty isn't just superficial though, the operation has deeper consequences. At the start of the book, Tally doesn't remember that she's endured the operation so the cure can be tested on her. When we catch up with her it's a month after she returned to the city and Tally is making the most of young pretty life.
This all changes after a visit from a past friend bringing the cure for her to take. Tally becomes "bubbly" (able to think clearly) rather than being "pretty-minded" and we see how she deals with living amongst the pretties when she knows the truth. We also learn more about how the world got to be the way it is in Tally's present.
Some of the best parts of this book are the different technologies of the future which are described really well and while futuristic they are completely believable. Also the pretty-talk which use by the characters shows how they are moving further away from being pretty-minded.
Reviews of the rest of the series: