Intelligence responds to a turf dispute that leaves two 17-year-old boys dead.
Interim Mayor Crawford tells Voight to run the investigation since the drugs they were dealing with was carfentanil, a drug 10x more potent than fentanyl.
Intelligence narrows it down to an independent drug dealer named Book Barr.
Upton ropes in her CI, who tends to get a little too geeked out about cases, to make the introduction.
During the buy, Barr tells Upton he won't sell to her unless she meets one of his guys.
Upton agrees and gets into the passenger seat of the car. On their way to meet the guy, she realizes it's a rip.
Barr crashes his car into the guy driving in front of him and as he's attempting to steal his drugs, a package explodes and he overdoses on the street.
Upton tells Cameron, her CI, to lie low for a little while as they take the case in a new direction but he doesn't stop.
Intelligence realizes that Darius, Voight's personal CI, has been working with Mackie and gave them Barr's name in the meantime.
Voight confronts Darius and warns him against lying since it's not part of his deal.
He arrests him and puts him in the cage until Darius talks.
He agrees to wear a wire to tap Mackie and delivers on his promise.
When he meets Mackie for the buy, he "pretends" to find a GPS tracker on Mackie's car and accuses him of being the snitch.
As he drives way, Intelligence rolls up and nabs Mackie.
Meanwhile, Cameron keeps trying to stay involved in the case and visits Upton in the police station.
While there, he sees Darius is being wired up.
Upton warns him that he could have been burned, but he's adamant that he's fine.
When she meets up with him later on, he is shot in the chest right in front of her. His blood splatters all over her face.
The man who popped him doesn't say much aside from a rehearsed confession, but Upton knows that Darius is responsible.
She wants Voight to take him off the case but he refuses.
There's too much at stake and Darius has proved himself valuable.
He tells Upton she's going to carry Cameron's death with her.