In a crossover with Chicago Fire, the Intelligence Unit hunt for an evasive serial arsonist.
Mayor Tom Kane finds out he has a serious medical condition and tries to focus on the upcoming gubernatorial primary.
The team work with a visiting detective to put an end to a gang's vicious tactics. Meanwhile, Voight learns that the body of his son's killer has been found.
As Zajac's campaign gathers momentum, Kane must adjust to the realities of living with his medical condition and how the disease will progress.
Kane and his camp battle with the media to dominate the situation when long-buried information about the disposal of toxic waste near the airport comes to the surface via a mysterious blogger.