In the distant future, man has conquered the galaxy. We have not encountered any other sentient species.
But we did create one.
On a space station between planets, geneticists created a new type of humans - the "Quaddies". Quaddies have no legs; An extra pair of arms exist where one would expect legs on a human. This difference makes it easier for Quaddies to work in zero gravity, where dexterity is more important than walking. Though sentient, the Quaddies have no rights; they are enslaved by their creators, who even decide with whom they may mate. Having known only servitude, most of them aspire to nothing more.
Trouble begins when Claire and Tony - a pair of young Quaddie parents - are informed that Claire must mate with someone else for her next scheduled pregnancy. They are in love, so they try to escape to the surface of the nearest planet - an environment for which they are completely unsuited.
The lovers are recaptured, but their plight is noticed by welding instructor Leo Graf. Leo takes up the cause of the Quaddies - arguing for their "human" rights and ultimately plotting their liberation.
Falling Free is chronologically the first book in Lois McMaster Bujold's famous Verkosigan Saga, although it takes place centuries before the birth of Miles Verkosigan, the protagonist and namesake of that series.
The book tackles issues of human rights and dignity and the morality of enslaving a people who are brought up never dreaming of freedom. The rights and responsibilities of the creators - those who mastered the genetic engineering that made possible the Quaddies - are questioned and resisted.
But it is also a love story and Bujold creates some interesting characters, even though many of them lack depth (the main villain is a corporate bad guy, driven only by profits and hate). The notable exception is Leo, who begins the book as an uninterested bureaucrat and transforms into an emancipator, willing to sacrifice his life for a people he did not even know existed a few months before.
Although it is not the first book Bujold wrote about the universe in which Verkosigan dwells, Falling Free is a good introduction to the series.