When District 21 goes after a Colombian drug gang following a series of brutal slayings, an asset in the investigation emerges.
Antonio believes a Colombian gang is involved in his son's kidnapping; Halstead learns new information about Lindsay; Voight offers Antonio a new way to get information about Diego's whereabouts.
Voight and the team check out a house that has been associated with a series of gangs; people from the past cause Halstead's temper to rise.
Olinsky tries to protect his daughter; a counterfeiting operation is investigated; when a hoarder is arrested, more than clutter is uncovered.
Atwater and Burgess must keep their new patrol car in mint condition; the Intelligence Unit uncovers drug smuggling; Voight's behavior toward Lindsay drives her right where he doesn't want her; tensions rise between Olinsky and Ruzek.
New York's SVU agents Fin and Rollins help track down a serial killer who rapes his victims; Burgess gets an opportunity to prove herself; Lindsay brings Halstead as her date to her high school reunion.
Voight's son may be involved with the Fitori murder case, intensifying tension between Antonio and Voight; Ruzek helps Burgess get retribution.
Voight's new Internal Affairs handler proves to be suspicious of his actions; Voight must work with his former partner from the Gang Unit; Halsted enlists Jin to help him gather more information on Lonnie Rodiger; Olinsky takes Ruzek down a peg.
Burgess and Atwater come across a gunshot victim and the only witness is Olinsky's daughter; Ruzek goes to Burgess for help with an undercover mission.
Halstead's badge is revoked when Lonnie Rodiger is found dead; Severide (Taylor Kinney) and Clarke (Peter Mills) find Dr. Elliot's torso; Sumner (Sydney Tamiia Poitier) tries to prove herself; Lindsay shares a moment with Severide.
Dueling gangs are suspected when a massacre occurs and $8 million is stolen; Platt asks Ruzek for help.
As Voight and his team search for the bombers, they realize there might be more bombs set to detonate; Burgess' niece fights for her life.
Cmdr. Perry notifies the unit that Pulpo has been released to help with a case; Ruzek gets difficult news; when a bit of Lindsay's past resurfaces, Voight reminds her how far she's come.
Platt and Voight discuss their pasts; Jin uses a scapegoat when he is caught tinkering with Voight's computer; Lindsay realizes she needs to come clean.