Chicago P.d. - Season 4 -

A crossover with Chicago Fire (Season 5, Ep. 9) where Kelly Severide is suspected of vehicular manslaughter.

Season 4 marked the end of an era for two of the show's original leads and introduced a future mainstay: Chicago P.D. - Season 4

The season begins with Commander Emma Crowley (Barbara Eve Harris) investigating Voight and Lindsay for the murder of Justin Voight’s killer, a dark secret that strains their relationship. A crossover with Chicago Fire (Season 5, Ep

Midway through the season (Episode 8), Antonio leaves the Intelligence Unit to join the State’s Attorney’s office as a chief investigator, a move designed to lead into the spin-off Chicago Justice . Midway through the season (Episode 8), Antonio leaves

The season's narrative is framed by accountability and the personal cost of life in the Intelligence Unit:

Part of a massive three-way crossover with Fire and Justice centering on the warehouse fire that killed Olinsky’s daughter.

Introduced in Episode 21 as a Detective from Robbery-Homicide, Upton eventually joins Intelligence, filling the vacancy left by Lindsay.