الموسم 4، الحلقة 1: Lt. Casey (Jesse Spencer), who is at the center of an undercover operation to bring down a trafficking ring run out of Nesbitt's (guest star Eric Mabius) strip club, finds himself in very dangerous territory. Meanwhile, as a result of the high turnover rate on squad, Severide (Taylor Kinney) gets a rude awakening when Capt. Dallas Patterson (guest star Brian White) is brought in, much to Boden's (Eamonn Walker) dismay. Dawson (Monica Raymund) confirms her suspicions that her life is about to change forever and Jimmy Borrelli (guest star Steven R. McQueen), the new candidate, makes an unusual first impression. Elsewhere, citizens of Chicago band together to take charge of their neighborhood in protest of a drug den in their area, resulting in ramifications that severely hinder members of Firehouse 51, who are called to the scene to battle a blazing fire.