Casey struggles with his personal life; the firehouse welcomes a new member; Boden clashes with the new assistant deputy commissioner.

A raging fire consumes a 25-story apartment building complex, trapping victims on the top floors.

Boden becomes skeptical of Gorsch's motives; Severide gets involved in a difficult situation following a child's rescue from a car wreck; Foster is tasked with giving Cruz personal advice.

A series of explosions hit Chicago; Herrmann adjusts to his new duties; Kidd is visited by an old friend.

Casey investigates the cause of a trailer park fire; Boden reaches his breaking point; Mouch, Herrmann and Cruz wonder why Molly's isn't living up to expectations.

Gorsch takes over Boden's morning meeting at Firehouse 51 and goes out on calls in his place; Brett and Foster respond to a call about a Halloween prank getting deadly serious.

Severide struggles to process his father's death; Brett thinks a teenage driver in a car accident is hiding something.

Firehouses 51 is called in to battle a structure fire that places everyone in peril.

A car pile-up impacts someone dear to the firehouse family. Brett and Foster encounter a harrowing call while Kidd and Severide find their relationship being tested.

Casey and Naomi escape a deadly fire in his apartment. Brett puts together a team to create a community fire safety presentation but they struggle to make it engaging. Severide and Kidd are at a crossroads.

Foster faces blowback after pointing out a doctor's oversight. Brett helps Casey look for a new apartment. Kidd prepares Tuesday the Dalmatian for the Fire Safety Dog Competition. A serious accusation is leveled against Severide.

When the firehouse is called to a motorcycle crash, Casey and Severide sense that the details don't add up. A rift develops between Mouch and Otis. Brett, Foster and Kidd work to secretly fix a dent in the ambulance bumper.

Cruz leads the charge in getting the firehouse to gear up for the Polar Plunge. Herrmann has a hard time bouncing back after dealing with a tough call. Foster's personal relationships begin to interfere with her job.

Casey struggles after experiencing a narrow miss on a call. Otis wins a unique sporting opportunity in a raffle. Grissom turns to Severide for assistance when the mayor's office sends a task force to Firehouse 51.

Tensions run high between Casey and Herrmann after the firehouse responds to an apartment complex fire. After discovering something extremely suspicious during the call, Kidd and Severide do some investigating and get CPD on the case.

A horrible storm wreaks havoc on Chicago and impacts Firehouse 51 when the power goes off. A little boy mysteriously shows up at the station and everyone works together to uncover who he belongs to. Foster and Kidd argue about Severide.

Severide and Boden search to uncover whether arson was the cause of a fire at a hair salon. Mouch, Herrmann, Otis and Kidd make a startling discovery while tracking down the missing piercing nozzle from Engine 27.

A serial arsonist cold case is reopened when Lt. Severide finds a connection to a recent salon fire. Firehouse 51 celebrates a retirement party.

Severide and Kidd continue to investigate Benny's old arson case. Unexpected news leads Brett to ponder a big decision. All hell breaks loose.