District 21 of the Chicago Police Department is made up of two distinctly different groups. There are the uniformed cops who patrol the beat and go head to head with the city's street crimes. And there's the Intelligence Unit, the team that combats the city's major offenses - organized crime, drug trafficking, high profile murders and beyond. These are their stories.
Intelligence investigates a gruesome case. Upon further examination, it appears the victim was wearing a CPD-issued wire at the time of his death. Meanwhile, Burgess meets her new partner, Julie Tay, who came from one of the worst assignments in the city. Together they respond to their first call, and Tay shares details of her past. Additionally, Commander Crowley confronts Lindsay about why she and Voight were at that location on the night that Intelligence pursued Justin's killer.
When a call comes in reporting a car being stripped, Burgess and Tay respond and discover a badly beaten young man and his fiancé missing. Fearing a public relations nightmare Voight and the team are on their best behavior as they search for answers and the missing woman. At the same time, Lindsay surprises Halstead with a big question. Commander Fogel berates Platt for going over his head to have Tay assigned to District 21.
When Platt goes to her father Robert's house for dinner, she warns him to be careful with his new girlfriend Natalie. Following dinner, Platt is physically assaulted as she's getting into her car and is rushed to Chicago Med. Voight, Lindsay and Halstead arrive at the hospital to investigate, and a concerned Mouch finds himself being questioned. Meanwhile, Antonio leads the rest of Intelligence at the scene as they look into previous threats against Platt, but his attention shifts when he is called into Robert's townhouse and finds signs of an even more gruesome attack on Platt's father.
When a car plows through a crowd of concert goers and gunfire erupts, Lindsay and Ruzek are on the scene. Unsure of the motive and concerned a terrorist group may be responsible, Voight orders in Homeland security, customs and the F.B.I. to assist. At the same time, Burgess spots Atwater's brother, Jordan, out where he shouldn't be. The team use their connections to throughout the city to bring some peace to the community before more blood is shed.
Intelligence is tasked with tracking down the dealers of fentanyl - a drug that has been linked to multiple overdoses around the city. As they investigate possible sources, the team uncovers a series of dealers in a college smuggling ring. Meanwhile, Tay and Burgess come across a bloodied child who exposes a dark and twisted secret about his home life with his parents. Additionally, Mouse causes tension as he looks to reenlist in the army, and Platt tells Tay that she has been reassigned to her original post.
After receiving a complaint from a frustrated neighbor, Burgess and Officer Sorensen arrive to the house of baseball player Jake McCoy, and find the dead body of a 16-year-old girl, Maya Collins. There is conflict within the case as Intelligence learns that Olinsky and McCoy are friends, and McCoy admits that he was helping Maya and other girls try to escape the sex trade. Meanwhile, Burgess and Sorensen track down a thief who turns out to be a cop with a rocky past, and Lindsay gets a mysterious flower delivery that may be tied to her past.
Voight calls the team to investigate a homicide involving the only witness in a rape and murder case against an extremely young man, Oliver Tuxhorn. Intelligence looks into Tuxhorn's family and their finances as they try to determine if someone was paid to take out the victim. Meanwhile, Stone offers Antonio a job as lead investigator for the state attorney's office, and Burgess and Sorensen's body cameras capture a bizarre exchange with a man that they pull over.
When a young officer is shot dead in the lobby of an apartment building, Intelligence looks to the latest known offenders who have threatened the police. But when a second officer is killed, the team finds that they are dealing with someone who is highly trained and knowledgeable. Voight works to keep the case contained as they deal with Chicago's ever-present war against the police. Meanwhile, Lindsay confronts Antonio about his new job with the State's Attorney.
Severide is brought into the District for questioning after his car is found to have caused a deadly crash. As the investigation gets underway, the team works to uncover a group of car-jackers who may be the key to Severide's innocence. Meanwhile, Voight pairs Burgess with Olinsky, who has a hard time feigning his excitement about this new partnership, and Lindsay considers meeting her estranged father who is now out of prison.
After the body of young woman is found with multiple gunshot wounds, Intelligence uncovers details of her case that are similar to other deaths. They work to link all of these murders together to pin down the serial killer, and appease the families of the victims who have been looking for answers. Meanwhile, Lindsay decides to meet her biological father for the first time, and Voight welcomes Kenny Rixton to the team when Ruzek takes an undercover assignment.
When a mutilated sex offender is found dead, it is unclear if he was the victim of a hate crime, sexual assault or self-mutilation. As Intelligence questions those related to the case, they find themselves up against a private "special police" force who may be covering up illegal activities. Meanwhile, Halstead runs Jimmy's DNA to confirm whether he is actually Lindsay's father.
After a female jogger is brutally murdered in the park, Rixton and Atwater arrive to see two suspects fleeing the scene. When the men make their way inside a church, the priest and parishioners gather outside to block law enforcement from entering. While investigating the case, Rixton gets caught in a lie with Voight.
The body of a 15-year-old girl is found crushed in an abandoned building after she runs away from a treatment center for troubled girls. Voight sends Halstead undercover as a guard to work from the inside at the center, where he believes the girls are trading sexual favors for something in return. Meanwhile, Platt feels guilty because the victim was brought into the District last year, and Platt doesn't remember her.