One of 51's own needs the support of the entire house to recover from a near disaster. Meanwhile, Herrmann (David Eigenberg) has to figure out a way to reopen Molly's after it gets shut down and Severide (Taylor Kinney) digs deeper into the arson investigation, while struggling to cope with one of his toughest days yet.
After a fire ruins a neighborhood wedding, Boden (Eamonn Walker) steps up to host the ceremony at the firehouse. That's much to the chagrin of his boss, Chief Riddle (guest star Fredric Lehne), whose political aspirations have caused friction from within the firehouse. Meanwhile, Dawson (Monica Raymund), who returns to the house, also butts heads with Riddle over her ability to perform and is forced to defend herself despite Boden's support. Severide (Taylor Kinney) finds himself fighting for his way back to his old position and Capt. Patterson (guest star Brian White) is caught between loyalty towards leadership and loyalty to his colleagues.
Members of the firehouse are frustrated with their inability to help Chief Boden (Eamonn Walker), whose job is on the line pending an internal investigation in the aftermath of a complicated call to a fire. Meanwhile, Lt. Severide (Taylor Kinney) struggles to accept his smaller role and adjusts to the presence of Capt. Patterson (guest star Brian White), his new commanding officer. Dawson (Monica Raymund) shakes things up on the work front and Casey (Jesse Spencer) attempts to find a crucial piece of evidence in his ongoing investigation against Nesbitt (guest star Eric Mabius). Elsewhere, Herrmann (David Eigenberg) struggles to keep the peace with Molly's newest neighbors.
The pressure on Firehouse 51 is at an all-time high as the internal affairs investigation into the handling of a recent fire picks up steam. Just when it appears that game-changing evidence comes to light that would serve to bail out Chief Boden (Eamonn Walker), new twists emerge that put his head right back on the chopping block. Brett (Kara Killmer) and Chili (Dora Madison) respond to a routine call and quickly find themselves in a life and death situation to help aid a stabbing victim who is bleeding out. Meanwhile, a stuck elevator summons the house to a high-rise hotel that tests everyone's mettle and candidate Jimmy Borrelli (guest star Steven R. McQueen) is conflicted when his older brother Danny (guest star Andy Ahrens) looks to get him transferred to a different house.
Lt. Casey (Jesse Spencer) puts his life on the line and digs deeper into the shady dealings of Jack Nesbitt (guest star Eric Mabius). With members of truck and squad at odds over Rice's (guest star Warren Christie) actions in the field, Chief Boden (Eamonn Walker) is left to figure out how to create unity in a divided house, while Severide (Taylor Kinney) 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 (Joe Minoso) to risk everything to save their own.
Lt. Casey (Jesse Spencer), who is at the center of an undercover operation to bring down a trafficking ring run out of Nesbitt's (guest star Eric Mabius) strip club, finds himself in very dangerous territory. Meanwhile, as a result of the high turnover rate on squad, Severide (Taylor Kinney) gets a rude awakening when Capt. Dallas Patterson (guest star Brian White) is brought in, much to Boden's (Eamonn Walker) dismay. Dawson (Monica Raymund) confirms her suspicions that her life is about to change forever and Jimmy Borrelli (guest star Steven R. McQueen), the new candidate, makes an unusual first impression. Elsewhere, citizens of Chicago band together to take charge of their neighborhood in protest of a drug den in their area, resulting in ramifications that severely hinder members of Firehouse 51, who are called to the scene to battle a blazing fire.
An apartment fire linked to a case of rape and attempted murder 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 (Mariska Hargitay) 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 (Taylor Kinney) to lay down the law and Dawson (Monica Raymund) puts her life on the line during a call.
At Sgt. Voight's (guest star Jason Beghe) urging, a determined Lt. Casey (Jesse Spencer) agrees to go undercover in an effort to provide proof of Jack Nesbitt's (guest star Eric Mabius) shady dealings. Meanwhile, Otis (Yuri Sardarov) and Severide (Taylor Kinney) continue to disagree about the integrity of Scott Rice (guest star Warren Christie), causing tensions to flare between members of truck and squad. Meanwhile, Herrmann (David Eigenberg) helps out a fellow member of the firehouse.
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 (Charlie Barnett) desperately working to save the life of a driver seriously injured in the wreck. Meanwhile, Casey (Jesse Spencer) has some doubts about Jack Nesbitt (guest star Eric Mabius), whose strip club he is remodeling on the side. Elsewhere, Hermann (David Eigenberg), Otis (Yuri Sardarov), Cruz (Joe Minoso) and Chaplain Orlovsky (guest star Gordon Clapp) help get their team ready for the peewee hockey championship, but Herrmann might prove more of an obstacle than the other team.
Lt. Severide (Taylor Kinney) lobbies Chief Boden (Eamonn Walker) for a favor for Scott Rice (guest star Warren Christie), an old friend needing a second chance with the CFD, but Casey (Jesse Spencer) and Otis (Yuri Sardarov) have their doubts. Elsewhere, Mills (Charlie Barnett) works to regain his active squad status while Brett (Kara Killmer) and Dawson (Monica Raymund) decide they need a big night out.
During a harrowing call to a structure fire, Herrmann (David Eigenberg) 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 (Taylor Kinney) makes up for lost time by retracing key friendships from his adolescence, while Mills (Charlie Barnett) discovers some surprising news. Elsewhere, Cruz (Joe Minoso) and Brett’s (Kara Killmer) relationship comes to a crucial juncture, while Otis (Yuri Sardarov) develops some unexpected feelings.
Interim Chief Pat Pridgen (guest star Matthew Del Negro) is brought over to temporarily fill in for Boden (Eamonn Walker), 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 (Eamonn Walker) becomes all too apparent. Meanwhile, Lieutenant Severide (Taylor Kinney) must deal with a most unwelcome addition to his squad, Brett (Kara Killmer) gets closer to a girl she previously rescued and Casey (Jesse Spencer) starts seeing a new woman, Beth (guest star Bree Williamson), but interesting complications quickly unfold.
Chief Pridgen (guest star Matthew Del Negro) comes under fire after making a tactical error during an apartment fire for which he throws Lt. Casey (Jessie Spencer) under the bus, leading Lt. Severide (Taylor Kinney) 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 (guest star Kenny Johnson), who is faced with the choice of whether to fit in with the firehouse or follow his superiors self-serving orders. Elsewhere, Mouch (Christian Stolte) 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 (Kara Killmer) and a reluctant Cruz (Joe Minoso) take Dawson (Monica Raymund) out on a double date and Boden (Eamonn Walker) focuses his efforts on his duties at home as a father.
More clues surface surrounding the fire that killed Shay, leading Severide (Taylor Kinney) to a most unexpected source who may be able to provide pertinent information. Meanwhile, Shay’s sister Megan (guest star Ellen Woglom) comes to town, prompting members of the firehouse to remember Shay’s life. Elsewhere, Cruz (Joe Minoso) is in for a surprise when Otis’s (Yuri Sardarov) grandmother moves into their apartment, while Boden finds a way to help his dying father (guest star Richard Roundtree) make the most of his last few weeks.
Mills (Charlie Barnett) and Brett (Kara Kilmer) 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 (Taylor Kinney) receives an interesting job proposal and Chief Boden (Eamonn Walker) finds himself at odds with his father (guest star Richard Roundtree) over his treatment and the planning of a big party in his honor.
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 (Jesse Spencer) and Dawson (Monica Raymund) have a hard time figuring how to juggle their personal and professional lives. Elsewhere, Chief Boden (Eamonn Walker) and Donna struggle with news that their newborn has potential complications and they receive a surprise out-of-town visitor.
Severide (Taylor Kinney) and Dawson (Monica Raymund) 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 (guest star Gordon Clapp) is the victim of a bad car accident. Elsewhere, Chief Boden’s (Eamonn Walker) father (guest star Richard Roundtree) visits, but quickly wears out his welcome with his unsolicited parenting advice.
When the team responds to a semi-truck leaking hydrochloric acid in a busy intersection, Casey (Jesse Spencer) and Dawson (Monica Raymund) find their relationship being put to the test. Meanwhile, Severide (Taylor Kinney) learns more about Brittany’s (guest star Serinda Swan) past and Mouch (Christian Stolte) has some difficulties in his dating life.
A house fire leads members of the truck and squad to race to save the lives of the couple trapped inside, and then are faced with the perplexing aftermath. Meanwhile, Chief Boden (Eamonn Walker) prepares to start his new family, while the combination of life at work and home pushes Casey (Jesse Spencer) and Dawson (Monica Raymund) apart. Elsewhere Molly’s II has its chance to shine at the Chicago Holiday Fest.
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, Lt. Severide (Taylor Kinney) brings in Sergeant Voight (guest star Jason Beghe, Chicago P.D.) and Detective Lindsay (guest star Sophia Bush, Chicago P.D.) into the mix for further investigating. Elsewhere, Mills (Charlie Barnett) wonders if the dangers of Newhouse’s (guest star Edwin Hodge) off-duty gig are worth the cash returns, while Molly’s II is in dire need of a good chef.
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 (Kara Killmer) is cornered by a dangerous man in the midst of the madness and Dawson (Monica Raymund) is shocked to discover what actually brought the chopper down.
Lieutenant Casey (Jesse Spencer) deals with the fallout of his sister Christie’s (guest star Nicole Forester) impending divorce. Meanwhile, Dawson (Monica Raymund) and Mills (Charlie Barnett) settle into their new roles at Station 51. Elsewhere, Brett (Kara Kilmer) 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 (Jesse Spencer) comes to the aid of his sister Christie (guest star Nicole Forester) and faces down his corrupt brother-in-law, while Brett (Kara Kilmer) goes above and beyond for a mother in need. Elsewhere, Mills (Charlie Barnett) gets educated on the exciting world of unlicensed private investigation and Dawson (Monica Raymond) begins training with Herrmann (David Eigenberg).
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 (Eamonn Walker) has a plan to get Dawson (Monica Raymund) her badge, but it comes at a cost. Elsewhere, Mills (Charlie Barnett) 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 (Monica Raymund) starts her first day as a candidate and looks to find a balance with Lieutenant Casey (Jesse Spencer) inside the firehouse. Mills (Charlie Barnett) must come to terms with a future that doesn’t include the squad. Meanwhile, Severide (Taylor Kinney) 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.
Boden (Eamonn Walker) forces members of Firehouse 51 to pull themselves back together in the aftermath of a harrowing fire explosion. When Severide (Taylor Kinney) goes missing for one week too many, Casey (Jesse Spencer) works to track down his whereabouts. Meanwhile, Newhouse (Edwin Hodge) brings some unexpected news regarding one of Mills’ (Charlie Barnett) relatives while Herrmann (David Eigenberg) and Otis (Yuri Sardarov) make some new goals regarding Molly’s.
Lieutenant Severide (Taylor Kinney) returns to Firehouse 51, but he is clearly not his usual self and Dawson (Monica Raymund) counts down the weeks until she gets started as a candidate at the Austin Firehouse. Meanwhile, Mills (Charlie Barnett) learns more about his father’s family and Herrmann (David Eigenberg) asks the rest of the Molly investors to come up with some new ideas to turn the business around.
With everyone’s support, Dawson (Monica Raymund) 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 (Jesse Spencer) has an important matter to discuss with Antonio (guest star Jon Seda, "Chicago P.D."). Meanwhile, Lt. Severide (Taylor Kinney) and the firehouse welcome Rick Newhouse (guest star Edwin Hodge), the latest addition to the squad, Shay (Lauren German) gets an uninvited surprise guest and Chief Boden (Eamonn Walker) makes a bold move.