Boden forces members of Firehouse 51 to pull themselves back together in the aftermath of a harrowing fire explosion. While Severide goes missing for one week too many, Casey works to track down his whereabouts. Newhouse brings some unexpected news regarding one of Mills’ relatives. Herrmann and Otis make some new goals regarding Molly’s.

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.

The members of Firehouse 51 and the Austin firehouse clash when trucks collide on the way to a call. Boden comes up with a plan to get Dawson her badge. Mills faces his past and future. The Molly's group hits a speed bump starting the food truck.

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 comes to the aid of his sister Christie and faces down his corrupt brother-in-law, while Brett goes above and beyond for a mother in need. Elsewhere, Mills gets educated on the exciting world of unlicensed private investigation and Dawson begins training with Herrmann.

In part one of the crossover with Law & Order: SVU and Chicago P.D., Severide returns from Vegas with a huge surprise that sends ripples through Firehouse 51. Mills wonders if the dangers of Newhouses' off-duty gig are worth the cash returns, while Molly's II is in dire need of a good chef. Members of Firehouse 51 are forced to contend with a house fire, but when the injured owner is rescued from his cellar clutching a suspicious box, Severide brings in Sergeant Voight and Detective Lindsay into the mix for further investigating. (Chicago Crossover (2) aired on Law & Order: Special Victims Unit, season 16, episode 7. They'll Have to Go Through Me (3) aired on Chicago P.D., season 2, episode 7.)

When a massive helicopter explosion in a densely populated area leaves a trail of debris scattered across a full city block, members of the firehouse spring into action as lives hang in the balance. Squad 3 finds itself in a precarious position, Brett is cornered by a dangerous man in the midst of the madness and Dawson is shocked to discover what actually brought the chopper down.

When the team responds to a semi-truck leaking hydrochloric acid in a busy intersection, Casey and Dawson find their relationship being put to the test. Meanwhile, Severide learns more about Brittany’s past and Mouch has some difficulties in his dating life.

A house fire leads members of the truck and squad to race to save the lives of the couple trapped inside, and then are faced with the perplexing aftermath. Meanwhile, Chief Boden prepares to start his new family, while the combination of life at work and home pushes Casey and Dawson apart. Elsewhere Molly’s II has its chance to shine at the Chicago Holiday Fest.

The firehouse receives unsettling news when two of their own come up missing following a routine call, prompting members of Chicago P.D. to lead the search. Meanwhile, Casey and Dawson have a hard time figuring how to juggle their personal and professional lives. Elsewhere, Chief Boden and Donna struggle with news that their newborn has potential complications and they receive a surprise out-of-town visitor.

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. Elsewhere, Chief Boden's father visits, but quickly wears out his welcome with his unsolicited parenting advice.

More clues surface surrounding the fire that killed Shay, leading Severide to a most unexpected source who may be able to provide pertinent information. Shay’s sister comes to town, prompting members of the firehouse to remember Shay’s life. Cruz is in for a surprise when Otis’ grandmother moves into their apartment. Boden finds a way to help his dying father make the most of his last few weeks. (A Little Devil Complex (2) aired on Chicago P.D., season 2, episode 13.)

Mills and Brett race to recover a missing teenager and are left stranded in the unforgiving elements, while the rest of Firehouse 51 babysits an abandoned newborn. Severide receives an interesting job proposal. Boden finds himself at odds with his father over his treatment and the planning of a big party in his honor.

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 linked to a case of rape and attempted murder calls for members of the CPD to team with the Special Victims Unit, which had a similar unresolved case that has eluded them for more than 10 years. This prompts Sgt. Benson to come in from New York to assist with the investigation in which she is all too familiar. Meanwhile, doubts among the squad towards one of their own causes Severide to lay down the law and Dawson puts her life on the line during a call. (The Number of Rats (2) aired on Chicago P.D., season 2, episode 20. Daydream Believer (3) aired on Law & Order: Special Victims Unit, season 16, episode 20.)

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.