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. Meanwhile, Newhouse brings some unexpected news regarding one of Mills’ relatives while Herrmann and Otis make some new goals regarding Molly’s.
Lieutenant Severide returns to Firehouse 51, but he is clearly not his usual self and Dawson counts down the weeks until she gets started as a candidate at the Austin Firehouse. Meanwhile, Mills learns more about his father’s family and Herrmann asks the rest of the Molly investors to come up with some new ideas to turn the business around.
Fingers get pointed and tempers flare between members of Firehouse 51 and the Austin firehouse when fire trucks collide en route to a perilous call. Meanwhile, Boden has a plan to get Dawson her badge, but it comes at a cost. Elsewhere, Mills confronts his past and his future on the same day and the Molly's crew hits some hurdles starting up their new food truck.
Dawson starts her first day as a candidate and looks to find a balance with Lieutenant Casey inside the firehouse. Mills must come to terms with a future that doesn’t include the squad. Meanwhile, Severide confronts his demons in an unexpected way and the Molly’s crew has a date with the city inspector to make sure their food truck is up to code.
Lieutenant Casey deals with the fallout of his sister Christie’s impending divorce. Meanwhile, Dawson and Mills settle into their new roles at Station 51. Elsewhere, Brett can’t decide if she wants to leave Chicago and the Molly’s crew still can’t figure out how to get the food truck started.
Casey comes to the aid of his sister Christie and faces down his corrupt brother-in-law, while Brett goes above and beyond for a mother in need. Elsewhere, Mills gets educated on the exciting world of unlicensed private investigation and Dawson begins training with Herrmann.
Members of Firehouse 51 are forced to contend with a house fire, but when the injured owner is rescued from his cellar clutching a suspicious box, Lt. Severide brings Sergeant Voight and Detective Lindsay into the mix for further investigating. Elsewhere, Mills wonders if the dangers of Newhouses's off-duty gig are worth the cash returns, while Molly's II is in dire need of a good chef.
When a massive helicopter explosion in a densely populated area leaves a trail of debris scattered across a full city block, members of the firehouse spring into action as lives hang in the balance. Squad 3 finds itself in a precarious position, Brett is cornered by a dangerous man in the midst of the madness and Dawson is shocked to discover what actually brought the chopper down.
When the team responds to a semi-truck leaking hydrochloric acid in a busy intersection, Casey and Dawson find their relationship being put to the test. Meanwhile, Severide learns more about Brittany's past and Mouch has some difficulties in his dating life.
A house fire sees the team attempting to save the lives of a couple trapped inside. Chief Boden prepares to start his new family. The combination of life at work and home sees Casey and Dawson drifting apart.