Lex lives down below, close to rock bottom. She grew up in the orphanage, alone, and now is training to be a Special Op so she can finally see the face of the enemy and destroy the rebels with her own hands. She needs no one.

Livia lives up top, miles above everything, on a floating island in the city of Indra. She is training too, but for a life that she does not want. What she wants is to be free, to finally leave her floating island and to run with her beloved horse, Veda, until she can't run any longer. But she can't let Governess down, or Marius. She must be a Proper Young Woman.

And then there's Kane. Lex's only friend, whom she would walk through fire for. And when she finds that Kane is in danger, she does not hesitate to leave her post and blast her way to the top of Indra to save him. She just needs to get one stubborn, unexpectedly clever airgirl named Livia to tell her where he is first.

In this fast-paced, dystopian thriller, Lex and Livia reluctantly team up and embark to save Kane after discovering that they share a mysterious identical mark, not realizing that their biggest danger is in each other. You see, these two girls have a secret, a secret that they do not even know: they are sisters. Twins. And that is forbidden.