The discovery of a body leads to a cache of weapons, which then leads Intelligence on a strange investigation into a team of "operatives" that are targeting criminals within the city. Roman treats a sick young boy to a day at the 21st district.
Intelligence tries to find a missing teenage boy they believe could be a threat to the students at his wealthy school when they find disturbing drawings in his notebook.
When a woman's head and decapitated body found in the Chicago River are connected to Voight's social group, Lindsay worries that his relationship with the group could affect his ability to see the case objectively.
Voight enlists his former cell mate to help take down a major cocaine supplier, while Burgess makes a move regarding her relationship with Ruzek and Roman comes up with a plan to fund Platt and Mouch's wedding.
The police investigate a boy's execution-style murder, which may be connected to another shooting. The incident also leads officers and members of the community to gather for a candlelight vigil.
The team looks for a suspect who held a woman for months, physically abusing her and psychologically attempting to manipulate her into forgetting her identity.
A murder investigation becomes personal for Voight when his son Justin is attacked in the same manner as a widowed, single mother whose body is found in the trunk of her car.