Voight, still cuffed, sits face to face with his new IA contact, Stillwell. This guy's song sounds familiar - he wants unbridled cooperation - but does he have more bite than Gradishar? He releases Voight with a threat for more jail time if things don't go his way.
While District 21 buzzes about Voight's arrest, a gang shooting sends Burgess and Atwater across town - and to the sole witness, Olinsky's teenaged daughter, Lexi. To keep her from testifying (and becoming a surefire target to the gang), Voight and Olinsky usurp the investigation from the Gang Unit and hunt for the likely suspect, a gangbanger named Calaca hooked up with the Latin Priests. But before the case gains traction, a foursome of executed Latin Priests are found in a field. This isn't just about territory anymore - this is an all-out gang war.
A tip leads Lindsay, Voight and Antonio to bust down the door of rival gang leader T-Mac, a member of the G-Park Lords with his eyes on killing Calaca. Once Lindsay chases him down, T-Mac caves to pressure from Voight and promises to stop murders. In other words, Intelligence buys some time to land Calaca. Problem is, when Lindsay and Antonio discover a front for PCP - the Latin Priests' drug of choice - they find irrefutable evidence of torture... including Calaca's pinky finger.
Ruzek ropes in Burgess for an undercover stint working as a naïve tourist in Millennium Park. Her goal: get a phone stolen by the Latin Priests so Jin can track it back to their secret headquarters. She nails the task with ease, but then her phone ends up as a dead end, stolen by a harmless old lady's foster son with a kleptomania problem. Later, Ruzek receives some distracting (and salacious) picture texts from his fiancée. While his guard is down, a thief snatches the phone and Ruzek chases him on foot, almost losing him on the CTA. When he relocates him, Ruzek follows the kid all the way back to the house he and Burgess visited earlier. Turns out that old lady supervises a group of foster kids who all steal phones for the Latin Priests. She reluctantly gives Ruzek an address to the gang's headquarters.
Intelligence raids the Latin Priests' warehouse and pulls Calaca into custody. But even Voight's untraditional form of interrogation goes nowhere - this guy's been arrested 15 times and never ratted on the gang. Unfortunately, that guarantees Lexi will have to testify if the case goes to trial. She reassures her worried father that, despite the danger, it's the right thing to do.
This contrasts with another call from Stillwell that brings Voight outside of town to a body, strangled to death. One look and Voight knows why he's there - it's Lonnie Rodiger, the kid who skirted a murder charge that Halstead obsesses over even after Voight's repeated attempts to warn him. Stillwell makes himself clear: Halstead better have a good alibi.
When Olinsky's daughter witnesses a gang murder, Intelligence rallies to protect her - and exact justice.