After he's injured in a terrible fire at an auto machine shop, Jose Vargas is forced to go on disability and leave his life as a firefighter behind - a task he finds incredibly difficult. Chief Boden tries to help a suspected teenage firebug. Also, Severide receives an enticing invitation from Renee, a woman who he saved from a car crash incident involving live electrical wires. Meanwhile, Cruz tries to save his younger brother Leon from the gang he is involved with.

Severide finds that he shares a familiar past with Eric Whaley, the Lieutenant who was brought into the firehouse to help out. Dawson lets Shay in on her secret, while Casey deals with family issues. Meanwhile, a bad batch of heroin on the streets has Boden and the team scrambling, while Clarice and Shay find they have a new problem to overcome.

Accused of sexual misconduct by a former paramedic candidate, Severide faces potentially dire consequences and fights for his survival both within the Chicago Fire Department and the legal system. Detective Voight returns, looking to collect on the favor that Dawson owes him for helping her brother, Antonio. Casey falls back into old patterns in his personal life, while Dawson, Herrmann and Otis become the proud owners of a real neighborhood bar as Molly's prepares for its grand opening.

Lieutenant Severide is anxious to implicate the serial arsonist, while Dawson and Shay get a shock on a routine call. Meanwhile, suspicions continue to point to Clarke as the mole inside the firehouse and Molly's falls deeper into the red. Elsewhere, Mouch campaigns for union president, with the aid of Isabella, and Boden butts heads with Gail McLeod.

Lt. Casey returns to work at Firehouse 51 after nearly being killed in a building collapse and Shay meets Allison Rafferty, her new partner, who is friendly to everyone except her. Dawson begins her rigorous training at the Chicago Fire Academy, while Lt. Severide prepares for his stint as instructor. Meanwhile, Clarke stubbornly does not speak with the police about the murder in which he is the prime suspect and the future of Molly’s looks bleaker than ever.

Lt. Severide hits the top of the list as Det. Lindsay starts calling in suspects with the clearest motives to make the guy who kidnapped Katie disappear. Meanwhile, Jones is still having trouble fitting in as word of her disruptions reach the fire department’s headquarters. Elsewhere, Herrmann celebrates his twentieth anniversary with his wife, but no one supports his idea on the gift he intends to give.