The police force is shaken after one of its own is targeted by a cop killer; Voight and team put everything on hold to identify the perpetrator from a long list of suspects and stop his violent rampage.
When a baseball player is the main suspect in a young girl's murder, Voight and team dig for the truth - and Olinsky, who has a history with the athlete, may be their secret weapon. Meanwhile, Lindsay receives flowers from an unknown party.
When an epidemic of heroin overdoses sweeps the city, Voight and team must trace the operation back to its leader; meanwhile, Mouse fights for the opportunity to rejoin the military, despite Halstead's attempts to convince him otherwise.
After a 16-year-old rapper is shot at his own concert, P.D. tracks social media in the hopes of preventing an all-out gang war; meanwhile, Atwater worries that his younger brother is at risk of being affected by gang violence.