Luke Garner is an illegal third child. All his life has been spent in hiding. Now, for the first time, Luke is living among others. He has assumed a deceased boy's identity and is attending Hendricks School for Boys, a windowless building with cruel classmates and oblivious teachers. Luke knows he has to blend in, but he lives in constant fear that his behavior will betray him. Then one day, Luke discovers a door to the outside. He knows that beyond the walls of Hendricks lie the secrets he is desperate to uncover. What he doesn't know is whom he can trust....

Nina Idi - a third child in a society where families are allowed only two children - has been betrayed by the boy she loved and arrested by the Population Police for exposing other alleged third children. Angry and confused, Nina knows only one thing for sure: She is innocent of the charges. But now she is faced with the most difficult choice of her life: Get three other prisoners to admit they are shadow children and be spared herself, or refuse to cooperate and be killed. The options are clear. The choice, Nina discovers, is not.

Luke Garner, an illegal third child, spent his first 12 years in hiding. For the past four months, Luke has lived among others, using the identity of Lee Grant, at the Hendricks School for Boys. But just as things are finally starting to go right, Lee's little brother, Smits, arrives at the school, and Luke finds himself caught in a tangle of lies that gets more complex with every passing day. Can Luke trust Smits to keep his secret? And can he trust Smits' menacing bodyguard, Oscar?

Trey may have saved Luke's life, but he still thinks of himself as a coward who can barely stand to be outdoors. Now Trey finds out Luke has been taken prisoner at Population Police headquarters. Trey is terrified, but he knows that if he doesn't rescue his friend, no one will. At police headquarters, Trey impersonates an officer to try getting to Luke. But just when it looks like he's close, Trey suddenly finds himself in danger of exposing not just himself but all shadow children. 

Matthias, an illegal third child, is caught in the crossfire between rebels and the Population Police. When he unwittingly saves a Population Police officer, Matthias is brought to Population Police headquarters to train as an officer himself. There he meets Nina, another third-born who enlists his help in a plot to undermine the Population Police. But Matthias is under constant scrutiny, and he has no idea whom he can trust. What can one boy do against a wicked bureaucracy?

Luke Garner is a third-born in a restrictive society that allows only two children per family. Risking his life, he came out of hiding to fight against the Population Police laws. Now, in the final volume of Margaret Peterson Haddix's suspenseful Shadow Children series, Luke inadvertently sets off a rebellion that results in the overthrow of the government. The people are finally free. But who is in charge now? And will this new freedom be everything they had hoped?