While alone at the station, Dawson is faced with an emergency that puts her training and confidence to the ultimate test.
An arsonist targets Lt. Severide. Meanwhile, Station 51 is under scrutiny by Gail McLeod, a consultant with the State Marshal’s office who is deciding if any firehouses should be closed. Mouch receives a visit from his Japanese girlfriend, Lt. Casey reflects on the one-year anniversary of Andy's death, and Shay has suspicions about Severide's paternity.
In the wake of Heather's incident, Lt. Casey is having a hard time handling his newfound responsibilities. Lt. Severide is on edge dreading the next attack from the firehouse's arsonist. Meanwhile, with Renee back from her mother's, Severide knows he has to confront her about his paternity suspicions. Elsewhere, Mouch begins his campaign for union president and Shay and Dawson feel that they need to get out more.
Severide tries to prove his suspicions about the arsonist's identity, while Casey gets accustomed to his new lifestyle. Molly's bar is in difficulties, Dawson goes on a date with a regular from the bar, and Mouch is on the campaign trail.
Lieutenant Severide is anxious to implicate the serial arsonist, while Dawson and Shay get a shock on a routine call. Meanwhile, suspicions continue to point to Clarke as the mole inside the firehouse and Molly’s falls deeper into the red. Elsewhere, Mouch campaigns for union president, with the aid of Isabella, and Boden butts heads with Gail McLeod.
Boden’s future with Firehouse 51 is on the line and McLeod keeps on him about his past wrongdoings, pushing him toward an early retirement. With Boden considering his options, McLeod calls on an unlikely candidate to potentially replace him as Chief. Meanwhile, it becomes clearer who the mole inside the firehouse is and everyone takes measures to make their disdain for him known. Meanwhile, Shay’s social life is in full bloom and Arthur continues to take money from the struggling Molly’s.
While out on a jog, Severide comes across a secluded and empty construction site and encounters a young boy who has been trapped in a backhoe loader and is in dire need of assistance. Boden and McLeod continue to butt heads about the future of the firehouse and Boden breaks news about his future to Casey. Meanwhile, it's election day for the union presidency and Mouch and Greg Sullivan have a debate on Otis' podcast. With rival bar Game Day's future in peril, the finger is being pointed at Molly's.
When a tanker collides into a commuter train that subsequently crashes into a warehouse, the firehouse is called to the horrific scene that tests everyone to the limits as life and death hangs in the balance. With the ultimatum in place for him or the firehouse, Boden is tasked with making life-changing decisions regarding his future. Severide learns more about Katie, the young woman he saw with his father. Meanwhile, Shay and Dawson are still at odds and Casey’s responsibility of being a surrogate father takes a sharp turn.
When the shooting of a young girl happens in their neighborhood, Cruz and his brother, Leon are forced to think about the gang they have tried to escape. The battle between Chief Boden and McLeod continues to escalate and Isabella convinces Mills to dress up for a fancy gala. Meanwhile, Lt. Casey is sad Dawson won't be coming by as much now that his parental duties are over, Shay is fed up with the firehouse and digs in deeper with Devon and Zoya's visa is expiring, prompting her to come up with an idea.
Chief Boden prepares to counterattack McLeod in a final effort to save the firehouse from closure and the romance between Dawson and Casey escalates. Leon is making progress in the gang murder investigation, but Cruz cannot stand being left in the dark by Voight, fearing for his brother's safety. Meanwhile, Severide and Katie bond to make up for lost time and Otis feels uncomfortable living with Severide and Shay after Devon's latest escapade.
All seems lost with firehouse 51 on the verge of closing its’ doors, thanks in no small part to Gail McLeod, but Chief Boden and the rest of the house refuse to allow themselves to sit by and let it happen. An emergency call to an apartment structure fire tests everyone’s limits and puts the squad in harm’s way. Meanwhile, Dawson receives some life-changing news and union president Greg Sullivan makes headlines, which has an immediate impact on Mouch.
Lt. Casey struggles on his return to Firehouse 51 with the injuries caused by his near-fatal accident. Shay's new partner - Alison Rafferty, isn't that polite. Dawson enrols at the Fire Academy to find a Lt. Severide is her instructor. Clarke doesn't co-operate when questioned about a murder case he is prime suspect in. Molly's future prospects continue to get worse.
Lieutenant Casey struggles to come to terms with his near-death incident in the line of duty as his memory loss becomes more apparent. Shay continues eluding a lawyer wanting to talk to him about Daryl's suicide as he comes into a large sum of money. Dawson notices new recruit Rebecca's test-taking abilities. Otis and Katie play a board game. A woman with a negligent landlord is helped out by Boden.
A drunk driver leaves a neighborhood without power when he drives into a transformer, leading to the team at Firehouse 51 to offer assistance on a cold night in Chicago. Most people from the community spend the night at the station leading to tempers rising and some romantic involvements heating up. Molly's bar leads to a dispute amongst it's stockholders about it's future and Casey continues to deceive both himself and Dawson about the condition he's in.
Chicago Blackhawk players Shay Seabook and Duncan Keith make friends with Clarke and end up helping out a case when Shay gets an unusual tip. Severide's sister Katie is missing and he feels sidelined as Detective Lindsay takes the case on. Casey finds out more about his head injuries. Dawson tries to pass a test.
Severide is getting impatient with the police as Vince Keeler walks free. Meanwhile, the house pulls out all the stops to save a girl trapped inside a donation bin, Mouch learns that Boden’s secretary Connie is not someone to mess with and the squad and truck are called to a serious high rise fire inside a 20-story downtown office building.
Lt. Severide hits the top of the list as Det. Lindsay starts calling in suspects with the clearest motives to make the guy who kidnapped Katie disappear. Meanwhile, Jones is still having trouble fitting in as word of her disruptions reach the fire department’s headquarters. Elsewhere, Herrmann celebrates his twentieth anniversary with his wife, but no one supports his idea on the gift he intends to give.
Lt. Severide is reminded of his own past by a man he saves from a car accident and feels compelled to help him. Elsewhere, Lt. Casey and Chief Boden grapple with how to dock Jones per her father’s orders. Meanwhile, Mouch and Dawson have troubles with the opposite sex and an emotional call to duty creates ripples throughout Firehouse 51.