The body of a fifteen-year-old girl is crushed inside a warehouse. She's identified as Sam Wells, a resident at Brady House, a home for troubled girls. When it's pointed out that the facility's number of calls to the police has dropped sharply in the last two and a half months, Voight is suspicious and sends Halstead undercover as a new security guard.
One of the other residents, Ellie, tries to come onto Halstead, telling him that the girls are trading sexual favors for other favors. When he later spots her with a facial wound she says reporting it would get her killed. Halstead gains her trust by revealing that he's an undercover cop and she tells him about a secret room in the facility that the girls use to get in and out.
The room is clearly the scene of Sam's murder, so Voight and the team formally come in along with dozens of uniformed officers. Ellie claims another resident named Tonya is behind the prostitution ring; Tonya says it's Ellie. Blood evidence and video footage suggest that Ellie was in the room when Sam died, but Halstead is unconvinced.
He and Lindsay return to Brady House and find Tonya's money and the phone she used to set up the appointments. The phone proves that Tonya killed Sam to stop her from speaking out. She is arrested, as is security supervisor Vince, who was her accomplice.
Brady House is shut down and Ellie is placed with a foster family in Libertyville, but not before she thanks Halstead.
Back at the district, Platt is shocked when Lindsay pulls a report that says Sam was brought in for a few hours previously; Platt can't remember her being there at all. She asks Voight to keep her in the loop on the case and sets out to find someone to claim her body. When she comes up empty, Platt claims the body herself. She and Voight carry out Sam's instructions for her burial, scattering her ashes into the water.