Among the Enemy is the sixth book in Margaret Peterson Haddix's seven-book Shadow Children series about a totalitarian society that forbade the birth of third children during a famine.
Like the fifth book, Among the Enemy features the trio of illegal third children friends Matthias, Percy, and Alia, who are now residing at the Neidler School. Unfortunately, early in the book, they are rounded up by the Population Police and strapped into trucks to be taken to a labor camp. However, using a nail to puncture the truck's tire, Matthias creates an accident that allows the three of them to escape though Alia is injurred in the accident. Running into the woods along the road carrying Alia, the three are soon caught in the Population Police's gunfire, and Percy is shot and injurred. Matthias finds a hideout for his friends, then runs to the cottage of their ally, Mr. Talbot, for help.
Mr. Talbot returns to the hideout with Matthias, but they both get caught in another round of gunfire. During the gunfight, Matthias saves Population Police Officer Tidwell, called Tiddy, who takes Matthias to Population Police headquarters, believing, based on Matthias's fake papers, that Matthias is a legal child. Though Tiddy soon dies of poison, the police commander rewards Matthias, allowing him to stay at headquarters. In headquarters, Matthias also meets third child Nina who, along with Luke Garner, is working in police headquarters as a spy to try and overthrow the Population Police.
Other adventures in the story involve Matthhias needing to rescue Nina, getting caught and rescued by a Population Police officer who says he is a double-agent, and later finding both Percy and Alia alive and well in Mr. Talbot's cottage.