Lt. Casey, who is at the center of an undercover operation to bring down a trafficking ring run out of Nesbitt's strip club, finds himself in very dangerous territory. Meanwhile, as a result of the high turnover rate on squad, Severide gets a rude awakening when Capt. Dallas Patterson is brought in, much to Boden's dismay. Dawson confirms her suspicions that her life is about to change forever and Jimmy Borrelli, 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.
Members of the firehouse are frustrated with their inability to help Chief Boden, whose job is on the line pending an internal investigation in the aftermath of a complicated call to a fire. Meanwhile, Lt. Severide struggles to accept his smaller role and adjusts to the presence of Capt. Patterson, his new commanding officer. Dawson shakes things up on the work front and Casey attempts to find a crucial piece of evidence in his ongoing investigation against Nesbitt. Elsewhere, Herrmann 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, new twists emerge that put his head right back on the chopping block. Brett and Chili 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 is conflicted when his older brother Danny looks to get him transferred to a different house.
One of 51's own needs the support of the entire house to recover from a near disaster. Meanwhile, Herrmann has to figure out a way to reopen Molly's after it gets shut down and Severide 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 steps up to host the ceremony at the firehouse. That's much to the chagrin of his boss, Chief Riddle, whose political aspirations have caused friction from within the firehouse. Meanwhile, Dawson, 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 finds himself fighting for his way back to his old position and Capt. Patterson is caught between loyalty towards leadership and loyalty to his colleagues.
Chief Boden meets his new neighbor, who asks him for a favor that leads to serious consequences. Severide takes offense to being treated like a candidate by Captain Patterson by undermining him during a call and the situation reaches a boiling point. Cruz receives a surprise visit from his past when Freddie, a gangbanger who knows Cruz's brother Leon, comes to visit and ask for a favor. Elsewhere, the team is called to a harrowing scene of an 18-wheeler and receive a thanks in the form of tickets to see the band Rush, much to Mouch and Herrmann's delight.
Chief Boden finds himself in the middle of a fiasco concerning a female neighbor, who he believes is setting him up. Severide, who is still on probation from his altercation with Captain Patterson, receives a surprise visit from his father and discovers some interesting information on Patterson that could be a game changer. Meanwhile, romance blossoms at the Chicago Fire Department gala.
Chief Boden confronts Maddox, believing he is the hand behind Serena's accusations intended to skew Boden's credibility from the approaching trial. Severide reaches out to Jamie, Maddox's lawyer, in an effort to help out and Capt. Patterson questions Boden's ability to remain Chief. Meanwhile, Brett and Chili discover a man passed out in a garden that may have a connection to a call from a previous crash site and Otis looks to fill up his social calendar, but the situation hits a bump in the road.
Members of Firehouse 51 are less than thrilled when Captain Patterson steps in as replacement for Chief Boden while he deals with clearing his good name. When Severide learns of the potential whereabouts of Serena, Chicago P.D's Officer Atwater and Officer Ruzek head to Lake Carlyle to try and uncover additional clues on Boden's case. Meanwhile, Herrmann and Otis give Freddie a job that keeps him close by and Jimmy and Chili elicit more attention then either bargained for, causing some friction between them.
Members of Firehouse 51 band together and anxiously await word at Chicago Med when one of their own falls victim to a life-threatening stabbing. Cruz urgently works with members of Chicago P.D. to track down Freddie's whereabouts when he goes missing. Dr. Choi and Dr. Halstead tend to an attempted suicide victim whose condition raises some eyebrows. Meanwhile, Mouch surprises Trudy and Chili's odd behavior has Brett and Jimmy worried.
A tornado threatens the city and Herrmann discovers the path may hit close to home. Chili and Brett lock heads when Chili makes a critical mistake that leaves a victim breathless, prompting Brett to report to Chief Boden regarding Chili's erratic behavior. Meanwhile, Otis pleads for Dawson to help him solve his women troubles and Severide is asked to lend assistance to agent Alex Ward from the Department of Homeland Security regarding a suspicious home.
Herrmann goes to court to testify against Freddie, but when the perpetrator's father reaches out to beg forgiveness for his son's malicious attack, a tormented Herrmann is left to figure out what to do. Following the aftermath of the recent tornado, Lt. Casey learns the makeshift shelter housing victims is on the brink of shutting down. At the request of Alderman Colin Becks, Casey attends a fundraiser and is thrown into the spotlight, only to quickly discover that his good intentions might not have the impact he was hoping for. Dawson and Brett feel the repercussions after telling Boden about Chili's recent erratic behavior. Meanwhile, Severide helps Agent Alex Ward solve a homemade explosives case and Otis receives a welcome surprise.
The firehouse is on high alert when mysterious threats begin to emerge throughout Chicago. The situation turns dire when news of a mass shooting brings members of squad and truck into the middle of a dangerous and harrowing scene. Lt. Casey works diligently to investigate the missing money from the fundraiser for the victims of the recent tornado. Meanwhile, Jimmy agrees to represent the firehouse in the first annual "Battle of the Badges" boxing match against the 21st district police department and Chili's erratic behavior continues to be a major issue for her ambulance partner, Brett.
Lt. Severide confronts Chili about his suspicions regarding her erratic behavior, but she does not take too kindly to his intentions. Meanwhile, members of the firehouse rally around Lt. Casey, who they believe is a perfect candidate to run for the position as Alderman and Jimmy faces off against Antonio in the first annual "Battle of the Badges" boxing match.
Firehouse 51 responds to an odd call from a neighboring firehouse and the unusual circumstances prompt Severide and Cruz to take the investigation into their own hands to search out the real story. The truck welcomes new firefighter Stella Kidd, but her addition has repercussion inside the firehouse. Meanwhile, Casey moves forward in his run for alderman, but begins to realize that some voters have their own personal agendas for wanting him elected.
Casey continues his run for alderman, but a citywide propaganda attack against his campaign makes him reconsider his candidacy. The rescue squad is summoned to the scene of a man trapped inside of a burning car with suspicious evidence suggesting that the fire may have been intentionally set. Mouch's upcoming bachelor party looms, but with Trudy's nerdy brother Logan handling the details, everyone fears for the worst. Meanwhile, Jimmy and Brett take a kidnapping investigation into their own hands.
While responding to a potential carbon monoxide leak inside a home, Lt. Severide and Cruz uncover the remains of a child inside the chimney. Subsequent information leads Severide to believe that the discovery may be linked to a case he had worked on years ago. Meanwhile, with election day nearing, Casey braces for the worst in his run for Alderman, prompting Dawson to enlist the firehouse team to lend a hand to the cause. Complicating his chances, Alderman Becks continues to bad mouth Casey to the press, but an incident in the field might prove beneficial
While responding to a call involving a victim incapacitated with a leg wound, Sylvie Brett witnesses a murder and is threatened by the shooter not to go to the police. Unsure of what to do, she tries to track down Antonio from Chicago PD. Elsewhere, Firehouse 51 jumps into action when a fire breaks out inside a restaurant, trapping several people inside a vault. Meanwhile, with his wedding day to Trudy fast approaching, a nervous Mouch starts to get cold feet.
When a high school student becomes an innocent victim of a gang retribution incident, a very determined Lt. Casey tries all means necessary to resolve the issue. Severide lends a hand to Detective Holloway, who is working a dangerous undercover investigation, and watches over her 9-year-old son at the firehouse. Brett and Jimmie are called to aid a woman who takes a fond liking to Jimmy. Meanwhile, Otis makes a surprising discovery that leaves him very concerned and Herrmann goes toe-to-toe with Stella to claim bragging rights over who can best run a bar.
Detective Holloway continues to entrust Lt. Severide to watch over her young son at the firehouse while she testifies in court as part of an undercover operation. When the situation takes an unexpected turn, Severide is left to deal with the aftermath. The firehouse responds to a blaze at an art gallery that has a victim trapped inside and prompts Cruz to make a rash decision amid the chaos. Casey receives a suspicious package in the mail that casts suspicion on him from the Board of Ethics. Meanwhile, Brett discovers Otis isn't taking care of his own important issues and confronts him, but finds he is stubborn and more interested in rallying the troops to organize a trip to Las Vegas for the annual firefighting conference.
Dawson makes an attachment to a young boy she rescues in an apartment blaze while Kidd's big idea for Molly's turns into a disaster. Lt. Severide makes a pitch for the house to upgrade some of their fire masks but hits a wall in his efforts, prompting Lt. Casey to use his political influence to pull a few strings. Meanwhile, Cruz and a reluctant Brett bring in a secret weapon in their efforts to get Otis to see Dr. Halstead and take a blood test.
Truck and squad are called to the scene of a building collapse that has trapped more than a dozen victims inside of the rubble. With the structure ready to give way and not much time to react, Chief Boden faces a difficult decision with life-and-death ramifications. In her quest to foster Louie, Dawson throws all her energies into the bureaucratic process necessary to make it happen, but it is the helping hand of a familiar face that gives her a fighting chance. Currently on medical leave, Otis swings by the firehouse and meets his temporary replacement, hoping it is just that but fearing the worst. Meanwhile, Severide and Kidd continue to see each other outside of the firehouse, and Casey and Antonio find themselves at odds and tensions flare.
In the Season 4 finale, Boden and Jimmy find themselves at odds in dealing with the aftermath of recent events while Kidd (guest star Miranda Rae Mayo) is forced to deal with her unstable and volatile ex, Grant (guest star Guy Burnet). With the help of firehouse 51, Dawson (Monica Raymund) tries to put her best foot forward in her pursuit for fostering Louie. At the urging of Susan Weller (guest star Lauren Stamile), his new political consultant, Casey (Jesse Spencer) tends to his Alderman duties and leaves town with her to attend a three-day summit. Much to his surprise, Casey embraces his new role while Susan urges him to consider grander aspirations. Meanwhile, the house responds to a dangerous structure fire that puts one of their own in a desperate situation.