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. Adding to the turmoil, Casey, unbeknownst to his colleagues, is in the midst of a separation from Hallie. 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.

When the drunk driver who hits a car and paralyzes a 16-year-old boy turns out to be the son of a dirty cop, Lieutenant Casey must decide whether or not to turn him in. Facing pressure and subsequent threats from the powerful and well-connected Detective Voight, Casey decides to tell the truth, inviting a stand-off. Additionally, Severide, who's fighting off the advances of the Chief's new assistant, learns he has a fractured vertebra in his neck that may never heal if he doesn't get treated, but ignores the doctor's advice so he can continue working. Meanwhile, Mills is torn between helping his mother at her restaurant and devoting himself full-time to the fire department, like his father did.