After a slew of brutal slayings, District 21, led by Sergeant Hank Voight go after a Columbian drug cartel cleaning house in Chicago. They discover D’Anthony who proves to be an asset in the investigation. At District, complicated histories and unit rivalries surface which could end up costing them one of their own. Meanwhile, Rookie Kyle Ruzek is pulled from the police academy by Olinski to join the team.

Antonio’s son was kidnapped and he has a feeling the Columbian drug gang is to blame. Pulpo, the Columbian gang leader, is being held in jail and suggests his release may trigger the release of Diego. During the investigation, Halstead learns some new information about Lindsay. Voight offers Antonio a new approach to get information on Diego’s whereabouts.

Voight and team head to a house that’s been associated with a number of top gangs. Events at the search result in Ruzek, being encouraged to seek some counseling which may have lasting repercussions professionally and personally. Meanwhile Halstead’s temper rises when he comes in contact with some unwelcomed people from his past.

More is learned about Halstead’s dislike of the Rodiger family and it’s family relationships all around as Olinsky shows a paternal side with his daughter Lexi in an attempt to protect her and as Voight’s trying to help his son who has recently been released from jail. Meanwhile the Intelligence Unit investigates a counterfeiting operation after discovering some high end cloth-like paper. Sergeant Platt sends Atwater and Burgess to arrest a hoarder where they discover more than just clutter.

Atwater and Burgess are given a new patrol car, but are challenged with keeping it brand new. A run in with a psychotic woman leads the Intelligence Unit to discover a drug smuggling operation which has put several girl’s lives in peril. Voight’s overprotective act with Lindsay leads her right where he doesn’t want as she also helps to keep Justin out of trouble. Meanwhile on stakeout, things get tense between Olinsky and Ruzek as Olinski tries to teach him the finer points of surveillance.

Voight and Antonio enlist the help of agents at the Special Victims Unit in New York, Fin and Rollins, to help track down the culprit of a series of rape/murders. Burgess has an opportunity to prove herself to Intelligence. Elsewhere Lindsay enlists the help of Halstead to act as her date for her high school reunion to get back at an old rival. The crossover starts on Law & Order: Special Victims Unit S15E15 Comic Perversion (I).

Voight's son may be involved with the Fitori murder case, intensifying tension between Antonio and Voight; Ruzek helps Burgess get retribution.

Voight is assigned a new Internal Affairs handler, Stillwell who proves to be suspicious of Voight’s actions. During an investigation involving a gambling ring Voight is forced to work with his former partner from the Gang Unit, Shi. At the same time Halsted remains fixated on Lonnie Rodiger and enlists Jin’s help to gather more information. Meanwhile, Ruzek’s inflated ego leads Olinsky to take him down a peg and offer a great opportunity to Atwater.

In responding to a call Atwater and Burgess discover a gunshot victim and the only witness is Lexi Olinsky. The shooting is tied to a Latin gang. To protect Olinsky’s daughter, Voight takes the case from the Gang Unit. Ruzek pulls up Burgess to help with an undercover mission that he hopes will help solve the case. Jon Seda, Sophia Bush, Jesse Lee Soffer, and Archie Kao also star.

Halstead's badge is revoked when Lonnie Rodiger is found dead; Severide and Clarke find Dr. Elliot's torso; Sumner tries to prove herself; Lindsay shares a moment with Severide.

Voight and the team discover a massacre, eight million in cash stolen and two wounded men in a clearing house. During questioning, one guy, Perko, provides some intel that is key to the case and leads Antonio to realize that two gangs were involved. Burgess steps up to help the unit and prove herself to Voight. Meanwhile, a softer light is cast on Platt when she enlists the help of Ruzek.

The Intelligence Unit works to find the bombers. Even with government agencies in the mix, Voight and his team work the case from their own angles and connections. Lindsay and Antonio question doctors who handled the transplant surgery for an Ambassador from Syria, but no clues emerge. Olinsky gets some solid intel and the team work from there. It’s a race against time as they realize there may be additional bombs targeted to detonate. Meanwhile Burgess sits vigil for her niece whose life hangs in the balance. The crossover starts on Chicago Fire S02E20 A Dark Day (I).