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.

In part one of the crossover with Law & Order: 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 crossover continues on Law & Order: Special Victims Unit S16E07 Chicago Crossover (II) and Chicago P.D. S02E07 They'll Have to Go Through Me (III).

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.