In the Season 4 finale, Lindsay's mother is a suspect in the murder of her boyfriend; and the case opens up when a group of high-school students OD, and the drugs in question relate back to the situation with Bunny. Lindsay also finds herself up against the review board dealing with consequences of her actions.
A live video is streamed online showcasing a tortured man being beaten and burned alive. Upon further investigation, the victim's identity is traced back to that of a man who had been previously locked up for statutory rape. When another attack of a similar nature occurs, Intelligence finds a connection between the crimes, suspected pedophiles, and group named the Perv Hunters. Meanwhile, Lindsay runs into a few bumps in the road both personally and professionally.
When the team is called to an in-progress bank robbery, they come across a Robbery-Homicide detective trying to take over the crime scene; and, as the robberies continue, Lindsay makes a decision that unveils a surprising suspect behind the mask. While working on the case at the district, Upton is shocked to come across Platt, who has a special connection to her past.
While investigating the murder of a teenage boy, Voight finds a connection with the murder weapon linking it to a case that he worked on 17 years ago. While the victim's father, a prominent defense attorney, tries to get the case taken over by Area Central Homicide, Voight digs back into the old case under the watchful eye of his former partner, Lt. Denny Woods.
After encouraging her sister to spend a night out with friends, Burgess wakes to find that Nicole never made it home. As the guilt begins to eat at Burgess, the team in Intelligence bands together to make finding Nicole their main priority. The case kicks into high gear when she is found clinging to life in an El station.
When a "night crawler" is murdered over a piece of mysterious footage, Intelligence investigates one of Chicago's wealthiest families and uncovers a lineage of old money and secrets. Meanwhile, Burgess's sister Nicole arrives at the District for a visit, and presents an icy attitude towards Ruzek,
After being tipped off about a woman running barefoot through the woods, Intelligence is led to a kidnapping victim locked up in a shipping container. The bizarre scenario leaves Voight questioning the legitimacy of the case, and sends the team on a twisted trail to get to the bottom of it. Meanwhile, a former relationship of Halstead's comes back into the picture, much to the surprise of Lindsay.
When Lindsay is tipped off about a father looking to avenge his daughter's death, Olinsky goes undercover as a hitman for hire; and Gang Intelligence reinvestigates the original case. Meanwhile, Ruzek is placed on patrol duty after returning from his undercover assignment.
Halstead goes undercover as a guard at a treatment center for troubled girls after one of its patients is found dead in an abandoned building; and Platt feels guilty because the victim was previously brought into the District, but she doesn't recall her.
The investigation into the murder of a sex offender leads to a private police force that may be involved in illegal activities. In other events, Halstead has Jimmy's DNA tested to see if he actually is Lindsay's father.
The team investigates a deadly crash involving CFD lieutenant Kelly Severide's car; Voight assigns Burgess with Olinsky; and Lindsay contemplates meeting her estranged father, who's been released from prison.
The team investigates the death of a witness in a rape-and-murder case; Stone offers Antonio a job in the state attorney's office; and Burgess and Sorensen's body cameras record a bizarre exchange with a man that they pull over.
Burgess and Sorensen discover a dead teen girl at the home of a baseball player who's friends with Olinsky; Burgess and Sorensen hunt a thief who turns out to be a cop with a rocky past; and Lindsay receives flowers.
The team hunts drug dealers selling fentanyl, which has been linked to multiple overdoses around the city; and Tay and Burgess discover a bloodied child who shares a twisted secret about his home life. In other events, Mouse contemplates reenlisting in the Army; and Platt informs Tay about her next assignment.
A car plows through a crowd outside of a concert venue and is followed by gunfire, sparking fears that it may be terror-related. In other events, Burgess spots Atwater's brother where he shouldn't be; and the team works to bring peace to the community before more blood is shed.
Platt is sent to the hospital after a vicious attack outside her father's house. Mouch is questioned and past threats against her are revisited while the team investigates; and a gruesome discovery inside the father's home ups the ante in the case.
Intelligence investigates when a young man is beaten and his fiancée is apparently abducted. In other events, Lindsay surprises Halsted with a big question; and Cdr. Fogel takes Platt to task for going over his head to have Tay assigned to District 21.
In the Season 4 premiere, the team investigates a murder in which the victim appears to have been wearing a CPD-issued wire; Burgess works a case with a new partner; and Crowley questions Lindsay about the night that Intelligence pursued Justin's killer.