No hay trabajo más estresante, peligroso o emocionante que el de los bomberos, rescatistas y paramédicos del Destacamento de Bomberos 51 de Chicago. Estos valientes se lanzan de cabeza al peligro cuando todos los demás corren en otra dirección.

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.

El jefe Boden es acusado de dejar morir a un vagabundo en el incendio de un depósito al ordenar a sus hombres que evacuaran el lugar, generando mala prensa y sentimientos encontrados en el departamento. Entretanto, el teniente Severide ayuda a una anciana.

Casey confronts Detective Voight about his harassing tactics, vowing never to back down, and ends up paying the price for his decision. Dawson faces disciplinary action for her on-the-job conduct. Meanwhile, Severide finds his arm failing him during a rescue at a construction accident, and reaches out to Anna, an old flame and pharmaceutical rep, hoping she might be able to help alleviate his injury problems.

Facing suspension, Dawson gets a hand from the same girl she saved through the procedures that landed her in front of the disciplinary board. Lieutenant Casey finds his fuse shortening as Detective Voight stops at nothing to get him to retract his statement. However, when Casey lays his life on the line during an apartment fire, he may have unknowingly caught the break he was needing.

Everyone in the firehouse is ordered to take a drug test after a call to a daycare center exposes them to a makeshift meth lab, and Severide stalls, not wanting his injury, or the painkillers he takes, to be discovered. Meanwhile, Truck and Squad find themselves pinned down by gunfire while trying to help the victim of a gang shooting, and a deep-fried turkey fire grows from small to explosive when a garage catches fire. Elsewhere, Dawson saves a bleeding man and Casey delivers a baby on the scene of a massive car pileup on the freeway.

At the site of a train accident, Peter Mills becomes deeply upset when he observes the dead body of a young victim. Unable to shake his emotional distress about the event, he questions his future as a firefighter. Dawson and Shay save a 6-year-old girl whose mother is abusing her – but their confrontation leads the woman to resort to dire actions. Meanwhile, Severide aggravates his injury while rescuing a heart attack victim and Shay sends Clarice, her married, pregnant ex-girlfriend, back home to her husband.

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.

Varios de los bomberos son acusados de robar un collar de diamantes después de responder a un incendio en una cocina en una casa lujosa. Mientras tanto, Cruz intenta salvar a su hermano de la participación en una guerra de bandas.

Cruz es perseguida por la culpa por la muerte de Flaco y se confiesa con Casey. Mientras tanto, Shay se recupera de un traumatismo craneal; Severide le pide ayuda a Dawson para poder hacerle frente a su lesión; Casey tiene problemas familiares, y Hermann tiene dificultades para aceptar un acuerdo de negocios.

Severide finalmente habla con el Jefe Boden de su lesión y toma decisiones personales importantes para su futuro.

El destacamento 51 es sacudido por acontecimientos que conducen a una muerte que sorprende a todos.

Severide se entera de que tiene un pasado en común con Eric Whaley, el teniente que trajeron al destacamento para ayudar.

Dawson intenta ayudar a su hermano Antonio a desentrañar los detalles de su investigación relacionada con drogas y pandillas y, a sus espaladas, recurre a una insólita fuente para pedir ayuda.

Cruz sufre de angustia emocional y termina poniendo en riesgo a sus colegas y a él mismo. Por otra paste, Casey, su hermana y su madre tratan de discutir a fondo sus problemas; y algunos miembros del equipo experimentan un cambio de perspectiva después de responder a una situación peligrosa.

Se abren viejas heridas emocionales cuando el padre del teniente Severide, Bennie, llega a la ciudad y choca con el jefe Boden por la muerte de su ex colega.

Con la situación todavía tensa entre el jefe Boden y Bennie Severide, la cosa estalla cuando Mills enfrenta a Bennie sobre el incendio en el que murió su padre.

Tragedy strikes when a rescue attempt at a tenement building proves unsuccessful, leaving the men and women of firehouse 51 affected by the loss of the victim. Severide's decision to put Mills on the fast track to move from Truck to Squad doesn't sit well with some of the others and tension mounts between Severide and Casey over Heather Darden. Shay and Dawson are forced to deal with a junkie's crazy behavior while on a call and Shay considers a different way to have a family. Meanwhile, the team draws the ire of local drug dealers who threaten the peace and safety of the neighborhood and firehouse.

Mills finds himself the target of resentment from some colleagues when Severide supports his desire to join Squad. Tara, a paramedic candidate, joins Dawson and Shay on the job for a few shifts. Meanwhile, Shay comes to the aid of Cindy Herrmann, who suffers from pregnancy complications while her husband Christopher is out of town. Elsewhere, Dawson and Casey draw closer as the grand opening of Molly’s draws near, and Casey receives a surprise visit.

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.

Events force Lieutenant Casey to team up with the Intelligence Unit, now run by Voight, who doesn’t appear to have changed his crooked ways after his stint in prison. Shay's excitement at the prospect of becoming a parent grows, while Mills and Dawson face some personal challenges. Meanwhile, the house comes together to support the opening of Molly's.

When a call comes in from the Cook County jail, the squad steps into a volatile predicament and are tasked with trying to defuse an escalating situation. Shay remains hopeful about her baby dreams and Hermann's wife Cindy goes into labor with their fifth child. Meanwhile, Mills and Dawson face key decisions in their personal lives.