In the Season 4 finale, Boden and Jimmy find themselves at odds in dealing with the aftermath of recent events while Kidd (guest star Miranda Rae Mayo) is forced to deal with her unstable and volatile ex, Grant (guest star Guy Burnet). With the help of firehouse 51, Dawson (Monica Raymund) tries to put her best foot forward in her pursuit for fostering Louie. At the urging of Susan Weller (guest star Lauren Stamile), his new political consultant, Casey (Jesse Spencer) tends to his Alderman duties and leaves town with her to attend a three-day summit. Much to his surprise, Casey embraces his new role while Susan urges him to consider grander aspirations. Meanwhile, the house responds to a dangerous structure fire that puts one of their own in a desperate situation.
Severide and Stella are on the lookout for a very unstable Grant, who left Chicago Med without notice. Now that Dawson is responsible for Louie, she questions her dangerous career path as a firefighter. Casey continues his work as Alderman but his relationship with his political consultant, Susan Weller, cools. Problems continue for Borrelli, who still blames Chief Boden for the death of his brother, and the situation reaches a boiling point. Meanwhile, Mouch and Brett start off on a new venture together.
Casey and Dawson find themselves unexpectedly caught in the middle of a potential political mess. Stella is left to make a serious decision about her ex-boyfriend. While responding to an auto accident, members of truck and squad find themselves in the middle of a dangerous situation. Elsewhere, Severide befriends a famous socialite that might distract him from his duties at 51, while Brett and Mouch set aside their writing differences for the greater good.
Casey enlists Susan Weller to help with an urgent personal matter. Severide receives an offer to travel luxuriously with rich party guy Travis Brenner and Stella uses all her resources to help a sick street kid after saving him during a tunnel fire. Meanwhile, Otis and Herrmann discuss their concerns over Molly's lack of customers.
While checking out a warehouse as part of his Alderman's duties, Lt. Casey finds himself in a bad spot when a forklift accidentally pierces a tank storing acid causing an extremely hazardous spill. With a father and daughter in peril, Casey is forced to use extreme measures to remedy the situation. Dawson and Brett respond to an elderly woman who has a difficult senior advocate. Meanwhile, Mouch and Brett continue along with their secret project together and Otis shares some exciting news as he prepares himself for an unusual expedition.
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