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 hug 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 returns to work at Firehouse 51 after nearly being killed in a building collapse and Shay meets Allison Rafferty, her new partner, who is friendly to everyone except her. Dawson begins her rigorous training at the Chicago Fire Academy, while Lt. Severide prepares for his stint as instructor. Meanwhile, Clarke stubbornly does not speak with the police about the hug in which he is the prime suspect and the future of Molly's looks bleaker than ever.
Lieutenant Casey tries to convince himself and everyone around him that he is one hundred percent healthy following his near-death incident in the line of duty. Shay, not wanting to delve into the emotionally traumatic incident, continues to elude a lawyer who wants to speak with her about Daryl's suicide. Meanwhile, Otis and Katie bond over board games, Boden helps out a woman with a negligent landlord and Dawson notices fellow recruit Rebecca Jones undertaking some interesting test-taking techniques at the Academy.
When a drunk driver knocks down a transformer, the entire neighborhood is left without power on a frigid Chicago night and members of Firehouse 51 step up to lend their much needed help. While many from the community ride out the night at the station, in the midst of such close quarters, romances speed up and tempers flare. Meanwhile, Casey continues to lie to both himself and Dawson about his condition and the stakeholders in Molly's bicker about the bar's direction.
Severide feels sidelined as he leaves Detective Lindsay and the Intelligence Unit to do their job and locate his sister Katie . Shay gets an unusual tip from Chicago Blackhawk players Brent Seabrook and Duncan Keith, who makes friends with Clarke and stop by Molly's. Meanwhile, Casey learns more about his head injuries after another close call and Dawson pushes hard to pass the physical test at the Academy.
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.
Lt. Severide sticks his neck out for Dave Bloom, a fellow firefighter from Denver who is troubled by addiction. Elsewhere, Dawson and Shay leave town for a "girl's weekend" at a cabin in the woods. Meanwhile Jones struggles with the knowledge that her father is pressuring Chief Boden about her career path and Mouch's internet dating profile gets an upgrade with the help of Mills and the guys.
A suicide note is delivered to a surprised Dawson and Lt. Severide digs deeper into Bloom's dark past. Elsewhere, Chief Boden learns shocking news, and with the help of Shay, Lt. Casey goes on the search for a life-changing item.
Lt. Casey and Dawson volunteer to help at a charity run at Chicago Medical when a massive explosion sends Chicago into chaos. With one of their own missing in the blast, the firehouse is tested like never before.
With everyone's support, Dawson makes final preparations to retake the fireman's exam, but a call to the field may throw a major wrench into her plans. Elsewhere, Lt. Casey has an important matter to discuss with Antonio. Meanwhile, Lt. Severide and the firehouse welcome Rick Newhouse , the latest addition to the squad, Shay gets an uninvited surprise guest and Chief Boden makes a bold move.
A fire at a boarding school leaves Lt. Severide shaken and wondering if he could have done more. Meanwhile, with the firehouse in full support of Dawson as she retakes the fireman's exam, Lt. Casey attempts some political maneuverings on her behalf. Elsewhere, Mills looks for answers about his father's mysterious past and Chief Boden's grand plans take a detour.