Severide returns to Firehouse 51, but he is clearly not his usual self. Dawson counts down the weeks until she gets started as a candidate at the Austin Firehouse. Mills learns more about his father’s family. Herrmann asks the rest of the Molly investors to come up with some new ideas to turn the business around.

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. Mills ponders an uncertain future. Severide faces his demons. A city inspector visits Molly's.

Casey deals with the aftereffects of his sister's divorce announcement. Dawson and Mills get used to their new roles at Station 51. Brett ponders whether to leave Chicago. The Molly's crew works to get the food truck started.

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.

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.

Two firehouse members are abducted, triggering a desperate search by the Chicago P.D. Elsewhere, Casey and Dawson struggle to balance their personal and professional lives, and Chief Boden deals with news that his newborn has potential complications.

Severide and Dawson unearth shocking evidence about the fire that killed Shay. Elsewhere, the firehouse bonds over a serious car accident, and Chief Boden's father visits, only to give unsolicited parenting advice.

Additional clues about Shay's death emerge. Meanwhile, Severide faces an old foe; Shay's sister arrives, inspiring the firehouse members to reflect on her life; Otis' grandmother relocates; and Boden sets out to help his dying father.

The crossover continues on Chicago P.D. S02E13 A Little Devil Complex (II).

An ice storm hits, forcing a citywide lockdown. Mills and Brett are left stranded in the elements while searching for a teen, and the firehouse members babysit a newborn. Elsewhere, Severide receives a job offer, and Chief Boden clashes with his father over his treatment.

A temporary chief fills in for Boden, who is forced to take some time off for personal reasons.

The interim chief makes a tactical error during an apartment fire, stirring up tensions and putting careers in jeopardy. Meanwhile, Mouch discovers that his donation to a fertility clinic has resulted in a child, and Dawson goes out on a double date with Brett and Cruz.

Severide lobbies Chief Boden for a favor for Scott Rice, an old friend needing a second chance with the CFD, but Casey and Otis have their doubts. Elsewhere, Mills works to regain his active squad status while Brett and Dawson decide they need a big night out.

Herrmann tries to rescue a toddler from a structure fire. Mills discovers unexpected news.

The Chicago fire team works seamlessly with Chicago Med personnel to help aid victims of a deadly gas leak, but things quickly take a turn for the worse when the hospital is suddenly forced into quarantine.

Truck and squad are called to the scene of a car crash that has Mills desperately working to save the life of a driver seriously injured in the wreck. Meanwhile, Casey has some doubts about Jack Nesbitt, whose strip club he is remodeling on the side. Elsewhere, Hermann, Otis, Cruz and Chaplain Orlovsky help get their team ready for the peewee hockey championship, but Herrmann might prove more of an obstacle than the other team.

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.

The crossover continues on Chicago P.D. S02E20 The Number of Rats (II) and Law & Order: Special Victims Unit S16E20 Daydream Believer (III).

At Sgt. Voight's urging, a determined Lt. Casey agrees to go undercover in an effort to provide proof of Jack Nesbitt's shady dealings. Meanwhile, Otis and Severide continue to disagree about the integrity of Scott Rice, causing tensions to flare between members of truck and squad. Meanwhile, Herrmann helps out a fellow member of the firehouse.

Lt. Casey puts his life on the line and digs deeper into the shady dealings of Jack Nesbitt. Chief Boden must figure out how to create unity in a divided house, while Severide comes to question his own judgment. A dangerous warehouse blaze quickly gets out of control and threatens members of firehouse 51.