S4, E13: The body of a 15 year old girl is found crushed in an abandoned building after she runs away from a treatment center for troubled girls. Voight (Jason Beghe) sends Halstead (Jesse Lee Soffer) undercover as a guard to work from the inside at the center, where he believes the girls are trading sexual favors for something in return. Meanwhile, Platt (Amy Morton) feels guilty because the victim was brought into the District last year, and Platt doesn't remember her.