A team of Chicago's toughest detectives are assembled to investigate an assassin with links to a Colombian drug cartel.
Lt Casey must work with Voight and the Intelligence Unit, Shay grows increasingly excited about the prospect of motherhood, and everyone comes together to support the opening of Molly's.
As the Intelligence Unit attempt to prevent a dangerous new weapon reaching the streets, a house search results in a traumatic incident for Ruzek.
As Officers Atwater and Burgess attempt to protect a new patrol car, a cocaine-fuelled assault leads them to a drug smuggling ring.
As internal affairs press Voight, a massacre at a Triad den brings him back into contact with a former partner.