P.d. - Season 4 - Chicago

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

In a devastating multi-episode arc, Alvin Olinsky (Elias Koteas) loses his daughter, Lexi, in a warehouse fire, pushing him to a grief-stricken breaking point.

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 . Chicago P.D. - Season 4

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

In "All Cylinders Firing," Desk Sergeant Trudy Platt (Amy Morton) is brutally attacked outside her father's home, leading to a personal and harrowing investigation into her past. Introduced in Episode 21 as a Detective from

Season 4 featured several "One Chicago" crossovers that connected the unit to Chicago Fire and Chicago Med :

The season introduces Lieutenant Denny Woods (Mykelti Williamson), Voight’s former partner turned rival, who begins a long-term campaign to take Voight down for past misconduct. 📺 Crossovers and Special Episodes A crossover with Chicago Fire (Season 5, Ep

In the season finale, Lindsay resigns from the Chicago Police Department to accept a position with the FBI in New York after being investigated for her conduct during an assault on a suspect. Off-screen, Bush cited abusive behavior and harsh working conditions as her reasons for leaving.