During an undercover operation, Lt. Casey lands in very dangerous territory; Dawson's suspicions are confirmed; a new candidate (Steven R. McQueen) makes an unusual first impression; a protest hinders firefighters who are trying to battle a blaze.
Chief Boden's job is on the line in the wake of a complicated call to a fire; Severide struggles to adjust to his smaller role and the presence of his new commanding officer; Casey works on finding evidence in his investigation against Nesbitt.
When new evidence appears in the internal affairs investigation that will clear Chief Boden, a twist puts him right back in the thick of it; Brett and Chili try to save a stabbing victim from bleeding to death.
Members of the firehouse band together to help one of their own; Herrmann looks for a way to reopen Molly's; Severide struggles to deal with one of his toughest days as he looks into the arson investigation.
Boden steps up to host a neighborhood wedding that was ruined by a fire; Chief Riddle's political aspirations cause friction at the firehouse; Severide fights his way back to his old position.
Chief Boden's new neighbor asks him for a favor; tension escalates between Severide and Captain Patterson; Cruz receives a surprise visitor; Mouch and Herrmann are excited to receive concert tickets to see Rush.
Boden thinks he's being set up by his neighbor; Severide's father pays him an unexpected visit; romance is in the air at the Chicago Fire Department gala.
Patterson questions Boden's ability to lead the firehouse as the trial approaches; Brett and Chili find an unconscious man in a garden; Otis tries to fill his social calendar.
Members of Firehouse 51 are unhappy when Patterson fills in for Boden while he tries to clear his name; Severide may have found Serena; Chili and Jimmy attract more attention than they expected.
A member of Firehouse 51 is treated for life-threatening injuries; Cruz searches for Freddie; Mouch has a surprise for Trudy; Chili's behavior worries Brett and Jimmy.
The city prepares for a tornado; Brett is forced to go to Boden about Chili's erratic behavior; Otis asks Dawson for help with his women trouble; the Department of Homeland Security asks Severide for assistance with a suspicious home.
Herrmann is tormented by a difficult decision; Casey is thrown into the spotlight at a fundraiser; Dawson and Brett are faced with the repercussions of their actions; Severide helps with a case regarding homemade explosives; Otis is surprised.
News of a mass shooting brings the members of Firehouse 51 into harrowing territory; Casey investigates the missing money from the fundraiser; Jimmy participates in a boxing match; Chili's erratic behavior continues to be a major issue for Brett.
Chili does not respond well when Severide tries to talk to her about her erratic behavior; members of the firehouse think Casey should run for the position of alderman; Jimmy and Antonio face off in a boxing match.
Firehouse 51 responds to an unusual call from another firehouse; the addition of a new firefighter (Miranda Rae Mayo) has repercussions; Casey realizes that certain voters have personal agendas for wanting him elected.
A propaganda attack causes Casey to reconsider his run for alderman; when a man is trapped inside a burning car, it seems the fire may have been set intentionally; Trudy's nerdy brother plans Mouch's bachelor party; a kidnapping investigation.
Severide is led to reexamine a case from the past while he investigates the discovery of a baby's body in a chimney; with election day nearing, Casey prepares for another confrontation with Alderman.
A call takes a bad turn when Brett witnesses a murder while aiding a victim who has a leg wound; a restaurant fire has people trapped inside a vault; Mouch gets cold feet about his upcoming wedding.
Casey attempts to use political influence to remedy a gang retribution incident; Severide helps Holloway in a dangerous undercover investigation.
Detective Holloway testifies in court as part of an undercover operation, leaving Severide to watch her young son; a fire at an art gallery with a victim trapped inside leads Cruz to make a rash decision.
Dawson risks her life to save a boy in an apartment fire; Severide pushes for an upgrade on the house's fire masks; Cruz and Brett have an idea to get Otis to take a blood test.
A building collapses and leaves dozens of people trapped and injured; Chief Boden faces a life-or-death decision in the rescue plan; Dawson continues her quest to foster Louie; tensions flare between Casey and Antonio.