Unlike the previous two books, Among the Betrayed features Nina Idi, the girl who was arrested along with Jason Barstow at the end of Among the Imposters. It is revealed that unlike Jason, who was a member of the Population Police, Nina was actually another third child who ended up becoming Jason's pawn.
Nina Idi, a third child, who wakes up in a jail cell. Having lived an extremely sheltered life with third elderly aunts and her grandmother as Elodie Luria, Nina reluctantly left a dreary but emotionally comfortable life to live at Harlow's School for Girls. After she met and fell in love with Jason Barstow, a boy at a nearby school, she was accused of trying to find third children in her school and arrested by the Population Police along with Jason.
Nina is given the two options after a frightening Population Police official interviews her: get three other prisoners to admit they are shadow children and be spared, or refuse to tell cooperate, and be killed immediately. Nina does not want to do either until they reveal that Jason betrayed her and he is actually a informant for their group. Emotionally torn, she is moved into a different cell containing the three other children; Matthias, Percy, and Alia, shadow children who are even younger than her, orphaned, and have no chance of survival.
As Nina cautiously befriends them, she struggles with Jason's betrayal and whether or not she should do the same. They manage to escape and find their way to Hendrick's School for Boys thanks to Matthias, Percy, and Alia's ingenuity and knowledge of the harsh world around them. They manage to live off the outdoor garden until they are caught by Lee Grant (Luke Garner) and Trey.
When the Population Police official arrives to arrest them, Nina, in a bid to save her friends, reveals everything to them; her real name and that she was supposed to betray them. The official reveals himself to be Mr. Talbot and that the whole ordeal was to test Nina's true allegiances. While she had convinced them of her innocence, she could only prove her trustworthiness by revealing she was supposed to betray Matthias, Percy, and Alia.
Though she has found some safety for now, Nina decides that she will help Lee and Mr. Talbot fight against the Population Police.
