Casey tries to deal with his unsettled personal life and the firehouse welcomes a new member. Boden clashes with new Assistant Deputy Commissioner Jerry Gorsch.

Firehouse 51 is faced with a daunting challenge when an out-of-control blaze consumes a 15-story apartment complex, trapping victims on the top floors. Complicating the rapidly escalating situation, members soon discover that someone close to home may be one of the trapped victims.

The crossover continues on Chicago Med S04E02 When to Let Go (II) and Chicago P.D. S06E02 Endings (III).

Chief Boden becomes increasingly skeptical of Assistant Deputy Commissioner Gorsch's motives, while Severide gets wrapped up in a difficult situation following the rescue of a child from a car wreck. Trying to fit in with the rest of the firehouse, Foster is tasked with doling out some personal advice to Cruz.

Firehouse 51 is on high alert when a series of explosions rock the city. Herrmann adjusts to his new duties and Kidd gets a visit from an old friend.

Working alongside a journalist, Casey does some digging into the cause of a trailer park fire. Boden hits his breaking point while Mouch, Herrmann and Cruz work to figure out why Molly's isn't performing up to expectations.

Halloween is in full effect for Foster and Brett, who are summoned to a home where a prank has quickly turned serious. Gorsch continues his reign of terror on Firehouse 51.

Severide faces some major adversity and Kidd is on a mission to help in any way she can. Following a car accident rescue involving a teenage girl and her young sister, a suspicious Brett has an unsettling feeling there is more to the story than what appears on the surface.

Firehouse 51 is called in to battle a dangerous structure fire that places everyone in peril. Severide makes a big personal realization. Casey lends another helping hand to Naomi, the investigative reporter.

Stakes are high when a massive car pile-up on the freeway impacts someone dear to the firehouse family. Brett and Foster encounter a harrowing call while Kidd and Severide find their relationship being tested.

A dangerous situation leaves Casey and Naomi in peril. Foster and Brett are in charge of planning a field trip visit to the station while Severide and Kidd go through tough times.

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 and work to determine exactly what transpired. A rift develops between Mouch and Otis. Brett, Foster and Kidd work to secretly fix a dent in the ambulance bumper they believe they unknowingly caused.

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.

Brett, Foster and Kidd take a road trip to Indiana and encounter an accident involving a boys' hockey team that leaves them scrambling and relying on their instincts to save the injured. The rest of Firehouse 51 competes in the annual chili cook-off.

A spate of robberies comes to light after a firehouse lockbox key goes missing; Sergeant Hank Voight suspects a firefighter may be involved, and with Boden's support, he recruits Cruz to go undercover to help sniff out any suspicious activity.

The crossover continues on Chicago P.D. S06E15 Good Men (II).

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.

Severide gets angry at Cruz when he feels his orders were disobeyed in the field. Boden reconnects with an old high school friend during a call to a house fire. Herrmann has issues when one of his favorite knick-knacks from Molly's goes missing.

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 in 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 and all hell breaks loose.

The mattress factory fire spirals out of control. Truck and squad lay it on the line in hopes of getting the trapped victims out alive.

Lt. Casey struggles to get Chief Boden to sign off on an energetic new recruit. Against her better judgment, Foster extends an olive branch to her irritating new medic. Otis inspires Herrmann to seek new partners at Molly’s. Brett realizes life in Fowlerton will take some getting used to.

Brett and Foster question the suspicious circumstances surrounding a brutal attack at a juvenile detention center, which leaves them searching for the truth. A convoluted new technology installed inside Firehouse 51 tests everybody’s patience. Kidd gets chosen to represent the district at a leadership conference.

A rare but deadly bacteria takes its toll on numerous victims around the city, leaving Chicago’s finest first responders to work together alongside the CDC to resolve the dangerous situation. Following a fire at a local university that ties into the outbreak, Severide has suspicions that something bigger is happening.

The crossover continues on Chicago Med S05E04 Infection, Part II (II) and Chicago P.D. S07S04 Infection, Part III (III).

Kidd must prove herself at the National Firefighter Leadership Conference while the rest of Firehouse 51 is called to two eerily similar car wrecks. Mouch tries to keep everyone in the firehouse informed by starting a newsletter, but things quickly get out of hand. Cruz deals with a heartbreak.

Cruz and Severide suspect foul play when a fire started by an elderly woman turns deadly. Kidd, Foster and Brett create a women’s-only lounge. Firehouse 51 celebrates some good news.

Severide must make a tough decision about the future of his career at Firehouse 51. Herrmann deals with the consequences of his temper. Cruz is excited to demonstrate the Slamigan at the Firefighting Expo. Commissioner Grissom returns.

Severide begins his assignment at the Office of Fire Investigation and gets off on the wrong foot when, despite his orders, he reopens an old case. When the cause of an apartment fire hits close to home, Herrmann is on a mission to get to the bottom of it. Kidd finds herself burning the candle at both ends.

An unexpected visit throws Casey for a loop. Severide sets out to exonerate a wrongly convicted arsonist. Kidd pays the price for burning the candle at both ends. Cruz goes on a mission to fix one of Otis’s old toys.

Members of Firehouse 51 are dismayed to learn their boundaries have been redrawn to overlap with an adjacent station and problems quickly arise. Casey and Gallo hunt a missing piece of equipment. Brett and Foster find themselves at odds. Mouch tries the find the recipient of a mysterious letter he discovers.

A pest control problem forces the 51 team to temporarily bunk with Firehouse 20. All under the same roof, Casey clashes with Capt. Delaney while Foster, Kidd and Brett fear Capt. Leone has it out for them. Boden mulls a procedural overhaul.

A series of false alarms at a private school hinders the firehouse's ability to respond to legitimate emergency calls. Kidd asks for ideas to surprise Severide on his birthday. Cruz and Brett rent out their extra room, but the new tenant is more than they bargained for.

The surprising arrival of a new fire truck brings both joy and chaos to members of Firehouse 51. Boden and Severide are asked to make peace with an old foe for the greater good. Brett receives family news while Foster plans a party with friends.

A series of mysterious gas leaks has members of Firehouse 51 puzzled and spread thin. Brett’s conflicted when her birth mother gets serious about tracking her down. Severide misunderstands his role as Cruz’s best man. Mouch bites off more than he can chew.

A series of teen opioid overdoses sees Severide partner with Sean Roman, but begins to suspect there’s more to the story than his old friend is telling him. Casey tries to be there for Brett as she considers her birth mother’s offer to meet.

The crossover continues on Chicago P.D. S07E15 Burden of Truth (II).

Boden is on edge after starting a new fad diet. Casey and Severide disagree on a call. Brett’s birth mother shares life-altering news. A familiar face has a proposition for Herrmann.

Things get personal for Casey and Gallo in the aftermath of a suburban house fire involving a mother and her young son. Herrmann campaigns for more respect. Kidd has an idea for giving back to her community.

Lt. Severide teams up with Seager and the OFI when a motel structure fire turns suspicious. Firehouse 51 members become amateur sleuths following a Reddit post involving one of their own. Brett continues to get invested in her birth mother, Julie.

Tension builds when a group of rabble-rousers attempt to lock down the firehouse. Brett is faced with a monumental decision. Cruz readies himself for a big day. Gallo and Ritter attempt to launch Firehouse 51 into the social media age.

When one of 51's own gets injured on a call, Lt. Severide becomes obsessed with helping out. Foster contemplates a big decision. Kidd is concerned for one of her high school trainees.