The Intelligence Unit investigates a gun-smuggling operation involving high-caliber bullets used in multiple murders. Ruzek has his first on-duty shooting, and Olinsky goes undercover to expose the gun running. Meanwhile, Halstead confronts someone from his past, and Voight helps a young boy escape a gang. Additionally, Erin consoles Kelly Severide when his sister goes missing, and Voight's son, Justin, is released from jail.