Tensions run high when trucks from two firehouses collide and fingers are pointed towards Cruz for his driving.

Dawson tries to strike a balance with Casey during her first day as a candidate. Meanwhile, Mills ponders an uncertain future; Severide faces his demons; and a city inspector visits Molly's.

Casey faces his corrupt brother-in-law and Brett goes above and beyond to help a mother in need. Meanwhile, Mills get an education on unlicensed private investigation and Dawson begins training with Herrmann.

During a house fire, Firehouse 51 rescues the owner from the cellar clutching a suspicious box, leading Lt. Severide to bring in Sergeant Voight and Detective Lindsey for further investigation.

The members of the firehouse spring into action when a helicopter explodes over a populated area.

Dawson and Casey's relationship is tested as the team responds to a truck leaking hydrochloric acid in a busy intersection. Severide learns about Brittany's past. Mouch has dating troubles.

Severide and Dawson discover startling evidence found at a storage unit fire that appears connected to the fire that took Shay's life. Meanwhile, the firehouse bands together when Chaplain Orlovsky is the victim of a bad car accident.

More clues emerge about the fire that killed Shay, leading Severide to a former nemesis; Shay's sister visits; Cruz is faced with a surprise; Boden tries to help his father.

The interim chief makes a tactical error during an apartment fire, stirring up tensions and putting careers in jeopardy.

Herrman tries to rescue a toddler from a structure fire; Mills discovers unexpected news; Brett and Cruz's relationship reaches a critical juncture; Otis is surprised by some feelings that develop.

Members of Firehouse 51 transport victims of a gas leak to nearby Chicago Med; an armed patient claiming to be carrying a deadly airborne disease forces Chicago Med into lockdown.

Mills tries to save a driver who's been seriously injured in a car accident. Meanwhile, Casey has doubts about the owner of a remodeled strip club and Hermann, Otis and Cruz prepare for the peewee hockey championship.

An apartment fire connected to a case of rape and attempted murder resembles an unsolved case in New York, so Sgt. Benson of Special Victims Unit comes to Chicago to help Intelligence with the investigation.