When Kelton is found murdered, Voight's grudge against the mayor-elect puts him under suspicion; Upton and Halstead are eager to prove their sergeant's innocence, but the more evidence they gather, the closer they come to doing the opposite.
When Atwater goes undercover in a drug syndicate, he learns there may be more to its kingpin than meets the eye; Halstead tries to join Atwater undercover, but finds himself at a disadvantage; Voight petitions the brass to show Ruzek leniency.
After a botched carjacking turns out to be something much bigger, Voight's unit goes toe-to-toe with a smart drug kingpin who only works with women; new recruit Vanessa Rojas struggles with the transition from under cover to intelligence.
After zeroing in on a suspect responsible for bioterrorism, the P.D. prepares to undertake its biggest manhunt yet; Upton is torn between helping track down the perp and providing emotional support to a scared young girl who needs her.
A murder in a Bulgarian neighborhood results in a challenging investigation when locals won't talk to police; Ruzek has a clash with a patrol officer.
The shocking murder of young boys affects Halstead; Crawford encourages Intelligence to use a new facial recognition software to help identify a suspect, but the plan backfires.
Voight pulls Darius in after Crawford asks intelligence to investigate a dangerous drug, but there is little trust in the relationship; when a plan to nail a dealer goes awry, the team must make a tough decision about Darius.
After injuring herself while pursuing a suspect, Burgess receives a shocking diagnosis; a homicide case turns into a missing persons case, with a sinister twist; Voight has to decide between justice and the letter of the law.
Halstead's secret ongoing relationship with a woman connected to another case puts him in jeopardy.
As it awaits news of Halstead's fate, the team turns its attention to an illegal arms dealer; Atwater discovers his brother is in Chicago and could be tied to the case; Voight must deal with Halstead's shooter; Burgess makes her decision.
Ruzek and Burgess' secret begins to have severe implications for their police work; a series of deadly acts of vandalism causes tensions to run high in the insular neighborhood of Canaryville; Atwater tries to reach out to his brother.
Voight makes a deal with Darius Walker to help bring down a rogue group of dirty cops dealing drugs that were supposed to be destroyed; Upton clashes with Voight and enacts her own form of justice.
After receiving a distressing 911 call, Burgess enlists the team's help to track down a girl caught up in a sex-trafficking ring.
A homeless man's murder leads Intelligence to a criminal who has evaded capture for years; Rojas has a troubling experience with a young mentally ill man.
Intelligence searches for the dealer who caused multiple overdoses and is connected to Roman's missing sister; the case becomes a homicide investigation when they find the dealer's dead body.
An armed robbery case quickly turns into a homicide investigation; when one suspect's background suggests domestic violence, Upton is forced to reckon with her childhood; Voight and Halstead butt heads over Halstead's righteous instinct.
Voight enlists the help of a reformed gang member to get a key witness' testimony in a murder case; more lives are put on the line as a gang war brews.
Intelligence does everything they can to build a case against the elusive leader of a drug ring, but things get complicated when Rojas realizes someone she cares about might be involved.
The clock is on when Ruzek witnesses a woman's kidnapping and the team races to put the pieces together; Burgess tries to connect with Ruzek outside of work, but he mysteriously distances himself from her.