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.
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.
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.
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.
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.
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.
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.
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.
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.
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.