Gabriela struggles between her crush on Matt and her budding romance with Peter. Kelly comes clean to the Chief about his arm; he also learns that the surgery's consequences might be even more severe than he'd thought. Casey's mother asks him to speak up for her at the upcoming parole hearing. Meanwhile, the calls of the day include a plane crash and a van full of illegal immigrants that were left to die.
After learning that an experimental surgery could significantly reduce his recovery time, Kelly starts having second thoughts about his decision to move to Madrid with Renée, and he asks his dad for advice. Matt speaks for his mother at the hearing. Chief Boden tries to save Ernie, the damaged kid, but it might be too late. Otis is sent to fill in for someone at another firehouse. Peter brings a puppy at the station. Also, Clarice comes back in Leslie's life.
One month later, Severide returns to the station after a full recovery, only to find out that the person who filled in for him is his ex-fiancée's brother. Otis reconsiders his decision to transfer. Herrmann, Gabby and him buy a bar. Casey's mother is released on parole and moves in with her son. Gabby sends a text to the wrong person. Clarice goes in labor. Also, there's a spike in heroin overdoses and Gabby's brother Antonio is determined to go after the guys who are behind it, no matter what it costs him.
In order to help her brother, Gabriela contacts an old enemy, even though this could ruin her friendship with Casey. Herrmann retaliates when Arthur, the silent partner from the bar, attempts to intimidate him. Leslie and Clarice try to avoid a custody lawsuit with Daniel. Severide tracks down his ex-fiancée, and Casey confronts his mother about the reason why she killed his father. The firehouse responds to a drowning kid and a collapsed house.
To make up for the crushing feeling of guilt, Cruz becomes overzealous at work, to the point where he puts a colleague's life at risk. Otis, Herrmann and Gabby find a hidden safe in their bar. Kelly finally confronts Renee Whaley. Casey meets with his sister to decide what to do about their mother, who has been acting quite recklessly. Meanwhile, Leslie struggles to prove she could be a fit mother to Clarice's baby, but to make matters worse, a homeless man sticks her with a needle, possibly exposing her to HIV.
After her break-up, Shay decides she wants to have a baby by herself. Gabby, Herrmann and Otis track down the owner of the box they found in the safe. At the scene of an accident, Peter Mills bravely talks a man out of suicide. Later, at the annual Academy Dinner, Gabby and Peter come out as a couple, Gabby overhears Benny Severide reveal a big secret about the death of Peter's father, and Andy Darden's widow surprises Casey.
Peter starts looking into his father's death and he confronts Benny Severide about it, while Chief Boden asks Gabriela to keep a secret. After an unexpected source tells her more about the sperm-donating process, Shay decides to take a different approach at having a baby. Kelly wants to help an old man whose restaurant has been put on fire as an intimidation method, but nothing is at it seems. Heather invites herself to dinner with Matt.
Matt is upset that Kelly wants Peter in the Rescue Squad. Further tensions arise when Kelly sees Matt with Heather and assumes he is sleeping with her. Kelly agrees to have a baby with Shay, but she cannot afford the artificial insemination, which leads her to a surprising decision. The firehouse gets shot at, and Casey follows a hunch in order to find the culprits. After attending a depressing funeral, Herrmann tries to reconnect with his family. The firehouse has a touching reaction to a tragedy in the neighborhood.
Casey is angry that Mills has been spending a lot of time with the Squad members and neglecting his Truck duties, but Peter believes Matt might be jealous on him and Gabby. While Herrmann is away hunting, his wife Cindy has a pregnancy complication. Shay and Severide have their first big fight as future parents. Tara, the new paramedic candidate, starts shadowing Shay and Dawson, but she doesn't seem to have what it takes for the job. She does however have a shocking surprise for Severide. Someone from Casey's past makes a comeback.
Severide has to face the consequences of Tara's accusations and the firehouse is ordered to attend a sexual harassment sensitivity seminar. Mills does a great job rescuing a man from an overturned tanker, which raises Hadley's envy. Shay and Dawson need to get past a poisonous snake in order to reach its victim. Matt and Hallie reunite, only to find out that their old nemesis Voight has been released from prison.
Severide decides to look into Tara's past to find evidence that would help him fight the criminal charges she filed against him. Peter asks Gabby to move in with him, but she feels burdened by the secret she's been keeping about his mother. A little girl with a special connection to the firehouse lights everyone's spirits up. The calls of the day include a collapsed apartment building, a car accident and a lightning victim. By the end of the shift, a personal tragedy hits one of the team members.
In the wake of Hallie's death, Casey must join forces with Sergeant Voight and his team in order to track down her killer, even though it means putting his own life in danger. Peter is still shaken after Gabby's confession and that might hurt their relationship. Leslie is very excited as she's about to get inseminated. Also, while preparing for the grand opening of Molly's, Otis convinces his partners to let him hire his Russian cousin as a waitress.
The team faces a hell of a ride when a small prison fire turns into a much more dangerous situation. Meanwhile, Cindy Herrmann goes into labor, Shay is anxiously waiting for her pregnancy test results and Mills faces a turning point in his love life and his career.
While out on a jog, Severide comes across a secluded and empty construction site and encounters a young boy who has been trapped in a backhoe loader and is in dire need of assistance. Boden and McLeod continue to butt heads about the future of the firehouse and Boden breaks news about his future to Casey. Meanwhile, it's election day for the union presidency and Mouch and Greg Sullivan have a debate on Otis' podcast. With rival bar Game Day's future in peril, the finger is being pointed at Molly's.
When the shooting of a young girl happens in their neighborhood, Cruz and his brother Leon are forced to think about the gang they have tried to escape. The battle between Chief Boden and McLeod continues to escalate and Isabella convinces Mills to dress up for a fancy gala. Meanwhile, Lt. Casey is sad Dawson won't be coming by as much now that his parental duties are over, Shay is fed up with the firehouse and digs in deeper with Devon and Zoya's visa is expiring, prompting her to come up with an idea.
Battalion Chief Boden prepares to counterattack McLeod in a final effort to save the firehouse from closure and the romance between Dawson and Casey escalates. Leon is making progress in the gang murder investigation, but Cruz cannot stand being left in the dark by Voight, fearing for his brother's safety. Meanwhile, Severide and Katie bond to make up for lost time and Otis feels uncomfortable living with Severide and Shay after Devon's latest escapade.
All seems lost with firehouse 51 on the verge of closing its' doors, thanks in no small part to Gail McLeod, but Chief Boden and the rest of the house refuse to allow themselves to sit by and let it happen. An emergency call to an apartment structure fire tests everyone's limits and puts the squad in harm's way. Meanwhile, Dawson receives some life-changing news and union president Greg Sullivan makes headlines, which has an immediate impact on Mouch.