Voight responds to a crime scene in which a young girl was hit by a stray bullet.
He helps to stop the bleeding until paramedics arrive before vowing to find out who did this.
Paul Staples from homicide arrived on the scene and told Voight he's got it from here, but Voight wasn't budging. This was his case now.
Staples attempted to establish authority, but again, Voight wasn't having it.
A deeper look into the events of the shooting revealed that Lamar Garrison was a key witness in the shooting of the intended target, gang leader, Brooks Campbell.
When Voight went to speak with Lamar, he pretended like he didn't know who the shooter was.
Voight reached out to his old friend and current Reverand who promised he'd talk to Lamar and get the name of the shooter in exchange for a sizeable donation to the church.
Lamar is hesitant at first, but tells Frank that JoJo, the leader of a rival gang, is responsible.
Intelligence arrests JoJo but find that his alibi checks out -- he was in a club at Boystown and doesn't want anyone to know about it.
The gang war between the rival gangs breaks out and Voight threatens to expose JoJo if he doesn't call his men off.
Eventually, Intelligence learns that Lamar's previous cellmate, Sean Atler, a white kid from the suburbs of Chicago, was the shooter. He was allegedly Lamar's best friend.
Staples learns that Lamar was planning to start pushing a variety of products that Campbell didn't agree on, so he staged a plan to get rid of him and keep his people's trust.
When Intelligence tracks down Atlet, they find he has a bullet to his head.
Lamar murdered his best friend to keep him quiet.
Voight loops in Frank and asks him to set up a drug deal so that they could catch Lamar in the act. Frank agrees, but Lamar is skeptical.
The meeting goes down at the church between Lamar, his people, Frank, and Atwater, who is undercover.
Lamar begins to disrespect Frank, so Frank hits back, which creates a tussle.
Voight orders his men to stand down as he believes Atwater has it under control. He emphasizes that he has a CI and an undercover cop inside, but Staples disregards his order and runs inside.
Everyone else is forced to follow suit.
When Lamar hears the sirens, he beats Frank to a pulp. Atwater can't react because he doesn't have a gun.
They arrest Lamar. At the precinct, Voight punches Staples after he tells him the only thing that mattered was getting the arrest.
Voight informs him that his CI has such bad brain swelling, he likely won't see another day.