When a fire claims one of their own, there's plenty of guilt and blame to go around. Lt. Matthew Casey, in charge of the Truck, tries to carry on, but butts heads with the brash Lt. Kelly Severide of the Rescue Squad - and each blames the other for their fallen team member. The firehouse also includes: Battalion Chief Wallace Boden, a fireman's fireman who is confronted by important personal decisions; paramedics Gabriela Dawson and Leslie Shay, who share a close bond and team together to face some of the most harrowing situations imaginable; Peter Mills, an academy graduate who is the latest generation in a family of firefighters; and Christopher Herrmann, a seasoned veteran who loses his home to foreclosure and now must uproot his family to move in with his in-laws.

Severide is hiding a serious shoulder injury sustained in the fire that killed Andy Darden, and grapples with his guilt as he fights to rescue a construction worker trapped in a building collapse. Adding to the burden, Darden's wife Heather turns on Severide at the annual firefighters' barbecue, blaming him for her husband's death. A call is made to an accident site at Hancock Tower, where two teenagers are pinned in a vehicle trapped beneath the falling debris of a broken window washer's cart. Meanwhile, Casey and Hallie work to figure out their future together.

Casey faces pressure to cover for a cop's son, who was responsible for a drunken-driving accident that left a teenager paralyzed. Meanwhile, Severide ignores his doctor's warnings that his neck injury may never heal if he doesn't seek treatment; and Mills must choose between working at the fire department full-time or helping his mom with her restaurant.

Old emotional wounds are opened when Lieutenant Severide’s father Bennie comes to town and clashes with Chief Boden over the death of their former colleague. Mills helps a distraught driver who caused an accident and Shay makes a decision about her future with Clarice. Meanwhile, Herrmann, Otis and Dawson get to the bottom of a mystery about their bar.

An arsonist targets Lt. Severide. Meanwhile, Station 51 is under scrutiny by Gail McLeod, a financial consultant for the city who is deciding if any firehouses should be closed. Mouch receives a visit from his Japanese girlfriend, Lt. Casey "celebrates the one-year anniversary of Andy's death, and Shay suspects Severide's paternity.

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. Newhouse brings some unexpected news regarding one of Mills’ relatives. Herrmann and Otis make some new goals regarding Molly’s.