The story of firefighters in Chicago.
Cast: Jesse Spencer, Taylor Kinney, Monica Raymund, Lauren German, Charlie Barnett, Yuri Sardarov, Christian Stolte, Eamonn Walker, Anne Lockhart, David Eigenberg, Joe Minoso, Randy Flagler, Don Kress, William Smillie, Steve Piper, Teri Reeves, Mo Gallini, Nicole Forester, Kathleen Quinlan, Anita Nicole Brown
When a call comes in from the Cook County jail, the squad steps into a volatile predicament and are tasked with trying to defuse an escalating situation. Shay remains hopeful about her baby dreams and Hermann’s wife Cindy goes into labor with their fifth child. Meanwhile, Mills and Dawson face key decisions in their personal lives.
Events force Lieutenant Casey to team up with the Intelligence Unit, now run by Voight, who doesn’t appear to have changed his crooked ways after his stint in prison. Shay’s excitement at the prospect of becoming a parent grows, while Mills and Dawson face some personal challenges. Meanwhile, the house comes together to support the opening of Molly’s.
A frustrated Severide, turns to an unlikely ally to resolve his current dilemma. Casey begins to find satisfaction in his personal life, while Mills and Dawson face personal challenges. Meanwhile, the squad is called to the scene of a harrowing apartment building collapse.
Detective Voight returns, looking to collect on the favor that Dawson owes him for helping her brother, Antonio. Casey falls back into old patterns in his personal life, while Dawson, Herrmann and Otis become the proud owners of a real neighborhood bar as Molly's prepares for its grand opening.
Mills finds himself the target of resentment from some colleagues when Severide supports his desire to join Squad. Tara, a paramedic candidate, joins Dawson and Shay on the job for a few shifts. Meanwhile, Shay comes to the aid of Cindy Herrmann, who suffers from pregnancy complications while her husband Christopher is out of town. Elsewhere, Dawson and Casey draw closer as the grand opening of Molly’s draws near, and Casey receives a surprise visit.
Tragedy strikes when a rescue attempt at a tenement building proves unsuccessful, leaving the men and women of firehouse 51 affected by the loss of the victim. Severide's decision to put Mills on the fast track to move from Truck to Squad doesn't sit well with some of the others and tension mounts between Severide and Casey over Heather Darden. Shay and Dawson are forced to deal with a junkie's crazy behavior while on a call and Shay considers a different way to have a family. Meanwhile, the team draws the ire of local drug dealers who threaten the peace and safety of the neighborhood and firehouse.
With tensions still running high between Chief Boden and Bennie Severide, things boil over when Mills confronts Bennie about the fire that killed his father. Meanwhile, Severide, Casey and Bennie uncover the truth behind two mysterious diner fires and Shay takes a new direction with her plan to have a baby.
Shay decides she wants to have a baby. Gabby, Herrmann and Otis track down the owner of the box they found in the safe. At the scene of an accident, Peter Mills bravely talks a man out of suicide. Later, at the annual Academy Dinner, Gabby and Peter come out as a couple, Gabby overhears Benny Severide reveal a big secret about the death of Peter's father, and Andy Darden's widow surprises Casey.
To make up for the crushing feeling of guilt, Cruz becomes overzealous at work, to the point where he puts a colleague's life at risk. Otis, Herrmann and Gabby find a hidden safe in their bar. Kelly finally confronts Renee Whaley. Casey meets with his sister to decide what to do about their mother, who has been acting quite recklessly. Meanwhile, Leslie struggles to prove she could be a fit mother to Clarice's baby, but to make matters worse, a homeless man sticks her with a needle, possibly exposing her to HIV.
In order to help her brother, Gabriela contacts an old enemy, even though this could ruin her friendship with Casey. Herrmann retaliates when Arthur, the silent partner from the bar, attempts to intimidate him. Leslie and Clarice try to avoid a custody lawsuit with Daniel. Severide tracks down his ex-fiancée, and Casey confronts his mother about the reason why she killed his father. The firehouse responds to a drowning kid and a collapsed house.
One month later, Severide returns to the station after a full recovery, only to find out that the person who filled in for him is his ex-fiancée's brother. Otis reconsiders his decision to transfer. Herrmann, Gabby and him buy a bar. Casey's mother is released on parole and moves in with her son. Gabby sends a text to the wrong person. Clarice goes in labor. Also, there's a spike in heroin overdoses and Gabby's brother Antonio is determined to go after the guys who are behind it, no matter what it costs him.
After learning that an experimental surgery could significantly reduce his recovery time, Kelly starts having second thoughts about his decision to move to Madrid with Renée, and he asks his dad for advice. Matt speaks for his mother at the hearing. Chief Boden tries to save Ernie, the damaged kid, but it might be too late. Otis is sent to fill in for someone at another firehouse. Peter brings a puppy at the station. Also, Clarice comes back in Leslie's life.
Gabriela struggles between her crush on Matt and her budding romance with Peter. Kelly comes clean to the Chief about his arm; he also learns that the surgery's consequences might be even more severe than he'd thought. Casey's mother asks him to speak up for her at the upcoming parole hearing. Meanwhile, the calls of the day include a plane crash and a van full of illegal immigrants that were left to die.
Casey sees his mom in prison.
An Internal Affairs investigation is launched when a woman accuses the firemen of stealing a diamond necklace from her home during a kitchen fire. The lead investigator is an old foe of Casey's. The Chief interrogates Shay and Dawson about some narcotics that went missing from the ambulance. Joe Cruz is faced with a big moral dilemma while trying to get his little brother out of a gang. Gabriela asks Matt out on a date, and things heat up between Kelly and Renée.
After inhaling magnesium shavings at a fire, Vargas gets a severe pulmonary injury, forcing him to retire from his job. Chief Boden fears that 14-year-old Ernie is behind several recent dumpster fires. Cruz's attempt to get his little brother out of a gang backfires. On a lighter note, Gabriela asks Matt to come over and repair her window, while Severide receives a dinner invitation from Renée, a very attractive woman that he saved from a car accident.
After seeing the remains of a victim crushed by a train, Peter is shocked and considers leaving the station. Gabriela attracts her superiors' wrath again, when she accuses a mother of poisoning her daughter. Kelly's addiction to painkillers spirals down, Leslie is confronted by Clarice's husband, and Mouch is very unfriendly to some Canadian visitors.
Between a meth lab explosion, a gang shooting, a turkey fire and a 10-car pile-up, the firehouse is very busy on Thanksgiving. Meanwhile, Chief Boden wants to help a damaged kid, Otis starts a podcast, Leslie's ex-girlfriend pays her a visit, Matt and Hallie have another talk about their future, and Severide risks getting in big trouble when the Chief orders everyone to take a drug test.
Gabriela prepares for her suspension hearing and Casey for his deposition. When it becomes obvious that Voight would do anything to protect his son, Hallie and Matt fear for their lives. Casey's only hope is that Det. Antonio Dawson can stop Voight before it's too late.
Kelly's arm problem worsens and he turns to an ex for help, as she happens to be a pharmaceutical rep. Gabriela risks suspension after hitting a patient's friend. Meanwhile, she bonds with Peter over food. Herrmann becomes involved in a pyramid marketing scheme. Also, Matt gets tired of Voight's harassment and decides to fight back, even though both Hallie and Chief Boden warn him not to do anything stupid.
A homeless man dies in a warehouse fire. Peter Mills might have saved him if he had another minute, but Chief Boden had called everyone outside, as the building was about to explode. Boden knows he did the right thing, but the victim's brother threatens to sue the city. Meanwhile, Det. Voight goes to the next level in his attempt to intimidate Casey; Kelly finds it suspicious that an old lady has been having several accidental fires lately; and Leslie learns some surprising news about an old girlfriend.
The firemen respond to a T-bone car accident that leaves a kid paralyzed. The drunk driver that caused the accident is the son of Det. Voight, a dirty cop who wants to cover it all up. However, Casey is determined to expose the truth, despite Voight's threats. Severide finds out what is causing his increasing arm pain; he also hooks up with Nicki and gets caught by her father. Also, Peter decides to quit his side job at his family's restaurant, but his mother is not happy with this career choice.
When a construction site collapses, a dying workman asks Severide to send his wife a message. Matt decides to work things out with Hallie. The firemen welcome Nicki, a charming young woman who will be working in payroll and who seems to have a thing for Kelly. The Truck members wonder why their crest is a goat.
Members of the Chicago Firehouse 51 are shaken after the loss of firefighter Andy Darden. Truck leader Matthew Casey and Rescue Squad leader Kelly Severide blame each other for Darden's death. However, they have to overcome the frictions when another colleague, Christopher Herrmann, is gravely injured. Meanwhile, Paramedic Gabriela Dawson performs a risky procedure in the field, in order to save a little girl's life; Casey goes through a rough patch with his fiancée Hallie; and the new candidate, Peter Mills, reports for duty.