Gianna Mackey joins the firefighters and paramedics team at Firehouse 51. Brett and Mackey handle a patient who suffers from a drug overdose as his brother threatens Brett with a gun. Boden suggests an idea that might have long-term consequences.
Brett and Casey suddenly find their lives in danger in the Ambulance 61 accident. Mackey becomes worried about her place in 51. Kidd deftly handles a firehouse problem. During a tense call involving a lady falling on the train tracks, Ritter puts himself in danger.
In the midst of a fire, a mistake on the aerial ladder leaves Mouch terrified, calling his abilities into doubt. While Kidd seeks encouragement from Severide, Casey and Brett speak about their future plans.
Boden unexpectedly runs into a man who suffers from dementia while on a mission assigned by the HQ. Upon saving Mackey's life in a building explosion, Gallo makes a quick decision. Mouch resurrects an old feud with a fellow firefighter from a nearby firehouse.
Herrmann and Cruz are stuck in an elevator with no way out when Firehouse 51 gets summoned to extinguish a fiercely burning fire inside a 10-story storage facility.
During a perilous fire rescue involving a woman covered in gasoline, Gallo makes another quick decision, whereupon Casey takes action because his orders were not followed. Following a series of odd phone calls, Brett and Mackey are on the lookout for answers.
Casey and Severide look into a suspicious blaze at a homeless camp. Cruz is rattled by a near-fatal accident. Ritter develops a strong relationship with a victim after the fire. Sylvie has a crush on Grainger.
Severide experiences conflicting emotions when he receives a call regarding a house fire which reminded him of an old memory. When Hermann is on vacation with his family, Casey finds that he got temporarily replaced by a person Brett is seeing, and he begins to vent his resentment on him.
While responding to a vehicle accident, Casey unwittingly becomes involved in a dispute with a drunk driver who causes Casey to suffer a head injury. Mouch, Mackey, Ritter, and Gallo receive a TRA training, during which Mackey is offered to be redeployed to another firehouse.
51 is summoned to investigate a spate of suspicious fires. Casey is still not feeling well after his head injury. As Mouch is hoping to achieve a long-held ambition, he enlists Gallo and Ritter's help.
Violet and Brett are tasked with tracking down a person who has sent them a cryptic message. When Severide discovers that a recruit is not following the rules while teaching a fireman class, he is forced to take action.
Hermann and Kidd assist a man who wants to be a fireman, only for them to discover that he has a criminal record. Casey begins to worry that he may leave the CFD.
A young lady contacts Stella at the fire station, claiming that she is in danger and that she and her brother are about to be killed at the hands of violent gangsters. Cruz competes in a baby swaddling match with Hermann.
Casey and Severide try to assist the owner of a pet food factory in finding the cause of a fire that broke out there. Everyone is distressed while preparing for the annual garage sale that takes place at 51.
After Stella gets promoted to Lieutenant, Boden, Severide, and Casey find themselves in an awkward situation. Meanwhile, Violet, Gallo, and Ritter organize a celebration for Mouch for earning a medal for valour and heroism.
Firehouse 51 receives some exciting news. An ambiguous visitor appears at Firehouse; meanwhile, Severide is taken aback when he receives a call about a boat that capsized in Lake Michigan.