Boden forces members of Firehouse 51 to pull themselves back together in the aftermath of a harrowing fire explosion. While Severide goes missing for one week too many, Casey works to track down his whereabouts. Meanwhile, Newhouse brings some unexpected news regarding one of Mills' relatives while Herrmann and Otis make some new goals regarding Molly's.
Lieutenant Severide returns to Firehouse 51, but he is clearly not his usual self and Dawson counts down the weeks until she gets started as a candidate at the Austin Firehouse. Meanwhile, Mills learns more about his father's family and Herrmann asks the rest of the Molly investors to come up with some new ideas to turn the business around.
Fingers get pointed and tempers flare between members of Firehouse 51 and the Austin firehouse when fire trucks collide en route to a perilous call. Meanwhile, Boden has a plan to get Dawson her badge, but it comes at a cost. Elsewhere, Mills confronts his past and his future on the same day and the Molly's crew hits some hurdles starting up their new food truck.
Dawson starts her first day as a candidate and looks to find a balance with Lieutenant Casey inside the firehouse. Mills must come to terms with a future that doesn't include the squad. Meanwhile, Severide confronts his demons in an unexpected way and the Molly's crew has a date with the city inspector to make sure their food truck is up to code.
Lieutenant Casey deals with the fallout of his sister Christie's impending divorce. Meanwhile, Dawson and Mills settle into their new roles at Station 51. Elsewhere, Brett can't decide if she wants to leave Chicago and the Molly's crew still can't figure out how to get the food truck started.
Casey comes to the aid of his sister Christie and faces down his corrupt brother-in-law, while Brett goes above and beyond for a mother in need. Elsewhere, Mills gets educated on the exciting world of unlicensed private investigation and Dawson begins training with Herrmann.
In part one of the crossover with Law & Order: SVU and Chicago P.D., Severide returns from Vegas with a huge surprise that sends ripples through Firehouse 51. Mills wonders if the dangers of Newhouses' off-duty gig are worth the cash returns, while Molly's II is in dire need of a good chef. Members of Firehouse 51 are forced to contend with a house fire, but when the injured owner is rescued from his cellar clutching a suspicious box, Severide brings in Sergeant Voight and Detective Lindsay into the mix for further investigating.
When a massive helicopter explosion in a densely populated area leaves a trail of debris scattered across a full city block, members of the firehouse spring into action as lives hang in the balance. Squad 3 finds itself in a precarious position, Brett is cornered by a dangerous man in the midst of the madness and Dawson is shocked to discover what actually brought the chopper down.
When the team responds to a semi-truck leaking hydrochloric acid in a busy intersection, Casey and Dawson find their relationship being put to the test. Meanwhile, Severide learns more about Brittany's past and Mouch has some difficulties in his dating life.
A house fire sees the team attempting to save the lives of a couple trapped inside. Chief Boden prepares to start his new family. The combination of life at work and home sees Casey and Dawson drifting apart.
The firehouse receives unsettling news when two of their own come up missing following a routine call, prompting members of Chicago P.D. to lead the search. Meanwhile, Casey and Dawson have a hard time figuring how to juggle their personal and professional lives. Elsewhere, Chief Boden and Donna struggle with news that their newborn has potential complications and they receive a surprise out-of-town visitor.
Severide and Dawson discover startling evidence found at a storage unit fire that appears connected to the fire that took Shay's life. Meanwhile, the firehouse bands together when Chaplain Orlovsky is the victim of a bad car accident. Elsewhere, Chief Boden's father visits, but quickly wears out his welcome with his unsolicited parenting advice.
More clues surface surrounding the fire that killed Shay, leading Severide to a most unexpected source who may be able to provide pertinent information. Meanwhile, Shay's sister Megan comes to town, prompting members of the firehouse to remember Shay's life. Elsewhere, Cruz is in for a surprise when Otis's grandmother moves into their apartment, while Boden finds a way to help his dying father make the most of his last few weeks.
Mills and Brett race to recover a missing teenager and are left stranded in the unforgiving elements, while the rest of Firehouse 51 babysits an abandoned newborn. Meanwhile, Lt. Severide receives an interesting job proposal and Chief Boden finds himself at odds with his father over his treatment and the planning of a big party in his honor.
Interim Chief Pat Pidgen is brought over to temporarily fill in for Boden, who is left to tend to his personal affairs. When tension is sparked as the result of his derisive leadership, the absence of Chief Boden becomes all too apparent. Meanwhile, Lieutenant Severide must deal with a most unwelcome addition to his squad, Brett gets closer to a girl she previously rescued and Casey starts seeing a new woman, Beth, but interesting complications quickly unfold.
Chief Pridgen comes under fire after making a tactical error during an apartment fire for which he throws Lt. Casey under the bus, leading Lt. Severide to take it upon himself to clear the problem. In an effort to further cover himself from any wrongdoing, Pridgen enlists the help of Lt. Welch, who is faced with the choice of whether to fit in with the firehouse or follow his superiors self-serving orders. Elsewhere, Mouch is floored upon learning that his donation to a fertility clinic has resulted in a child who, now a young adult, wants to meet him. Meanwhile, Brett and a reluctant Cruz take Dawson out on a double date and Boden focuses his efforts on his duties at home as a father.
Lt. Severide lobbies Chief Boden for a favor for Scott Rice, an old friend needing a second chance with the CFD, but Casey and Otis have their doubts. Elsewhere, Mills works to regain his active squad status while Brett and Dawson decide they need a big night out.
During a harrowing call to a structure fire, Herrmann rescues a trapped toddler, but the questionable circumstances surrounding the event sends him on a personal mission to locate the father and get to the bottom of the situation. Severide makes up for lost time by retracing key friendships from his adolescence, while Mills discovers some surprising news. Elsewhere, Cruz and Brett's relationship comes to a crucial juncture, while Otis develops some unexpected feelings.
It's all hands on deck when Firehouse 51 races to the scene of a deadly chemical leak inside an industrial building. With squad and truck tested like never before, scores of workers are rescued but many are experiencing respiratory distress. This prompts truck 81, squad 3 and ambulance 61 to spring into action to transport the victims to nearby Chicago Med. The situation takes a turn from bad to worse in the midst of the chaos when a "patient" planted inside the hospital goes on a suicide mission armed with a grenade and claiming to be the carrier of a deadly airborne disease. Trying to stop the madman from engaging the explosive device, a member of the firehouse team is severely wounded in the blast and left to face life-threatening injuries. With no other choice, Chicago Med is forced into lockdown until doctors and the Hazmat team can determine the severity of the airborne virus.
Truck and squad are called to the scene of a car crash that has Mills desperately working to save the life of a driver seriously injured in the wreck. Meanwhile, Casey has some doubts about Jack Nesbitt, whose strip club he is remodeling on the side. Elsewhere, Hermann, Otis, Cruz and Chaplain Orlovsky help get their team ready for the peewee hockey championship, but Herrmann might prove more of an obstacle than the other team.
An apartment fire linked to a case of rape and attempted hug calls for members of the CPD to team with the Special Victims Unit, which had a similar unresolved case that has eluded them for more than 10 years. This prompts Sgt. Benson to come in from New York to assist with the investigation in which she is all too familiar. Meanwhile, doubts among the squad towards one of their own causes Severide to lay down the law and Dawson puts her life on the line during a call.
At Sgt. Voight's urging, a determined Lt. Casey agrees to go undercover in an effort to provide proof of Jack Nesbitt's shady dealings. Meanwhile, Otis and Severide continue to disagree about the integrity of Scott Rice, causing tensions to flare between members of truck and squad. Meanwhile, Herrmann helps out a fellow member of the firehouse.
Lt. Casey puts his life on the line and digs deeper into the shady dealings of Jack Nesbitt. With members of truck and squad at odds over Rice's actions in the field, Chief Boden is left to figure out how to create unity in a divided house, while Severide comes to question his own judgment. A dangerous warehouse blaze quickly gets out of control and threatens members of firehouse 51, prompting Boden and Cruz to risk everything to save their own. Elsewhere, Dawson gets some life-changing news.