Phaet Theta has lived her whole life in a colony on the moon. She's barely spoken since her father died in an accident nine years ago. She cultivates the plants in Greenhouse 22, lets her best friend talk for her, and stays off the government's radar. Then her mother is arrested. The only way to save her younger siblings from the degrading shelter is to enlist in the militia, the faceless army that polices the lunar bases and protects them from attacks by desperate Earth dwellers.
Hiding in plain sight with her friend Wes' family on the drowned planet Earth, Phaet discovers the rugged beauty of the world her ancestors once called home. All her life, she's been taught to fear the "Earthbound", and their generosity and kindness surprise and touch Phaet. But when the Lunar Bases attack Wes' isolated village, Phaet's past catches up with her, and she's forced to choose: stay on Earth and fight beside the boy she's falling for or stow away on a Moon-bound ship so she can save her brother and sister from the government that killed their mother.