Best friends Kayla and Mishalla know they will be separated when the time comes for their Assignments. They are GENs, Genetically Engineered Non-humans, and in their strict caste system, GENs are at the bottom rung of society. High-status trueborns and working-class lowborns, born naturally of a mother, are free to choose their own lives. But GENs are gestated in a tank, sequestered in slums, and sent to work as slaves as soon as they reach age fifteen.

When Kayla is Assigned to care for Zul Manel, the patriarch of a trueborn family, she finds a host of secrets and surprises-not least of which is her unexpected friendship with Zul's great-grandson. Meanwhile, the children that Mishalla is Assigned to care for are being stolen in the middle of the night. With the help of an intriguing lowborn boy, Mishalla begins to suspect that something horrible is happening to them.

After weeks of toiling in their Assignments, mystifying circumstances enable Kayla and Mishalla to reunite. Together they hatch a plan with their new friends to save the children who are disappearing. Yet can GENs really trust humans? Both girls must put their lives and hearts at risk to crack open a sinister conspiracy, one that may reveal secrets no one is ready to face.