Lindsay and Ruzek are working overtime by providing security after a concert when someone starts shooting into the crowd. A black SUV tears through injuring several people, and Voight immediately calls in every agency possible as he suspects terrorism.
Halstead tracks the SUV to a man named Greg who has a prior record for a hate crime but Greg claims his car was stolen. Social media then shows the attack may be part of a feud with the sixteen-year-old rapper performing, who's a member of the South Side Hustlers gang.
Another shooting is called in right after, and while the team is responding to that second call video of a third shooting hits the Internet. Voight officially declares that it's not terrorism but a full-blown gang war between the Hustlers and the G Park Lords.
When tech support follows the social media thread Ruzek theorizes that someone wanted the war to happen. Olinsky and Dawson zero in on a suspect who belongs to a third gang, the Latin Players. But Voight thinks that man was working for someone and gets a name and location from one of his own gang sources.
Meanwhile, Burgess finds Atwater's brother Jordan at the home of one of the possible suspects. When Tay tells Atwater it does not go over well. Between that and having been the officer to save the wounded rapper he loses his cool and gets into a fight with one of the gang members while at Chicago Med.
When Voight's tip leads to the teenage shooter Alex Santos, he's arrested and Voight summons the leaders of both gangs to tell them the truth, restoring peace to the neighborhood. And Atwater struggles with his brother being too close to the wrong people.