Chicago Fire celebrates its 100th episode with a wedding, an anniversary, and an adoption problem.

Casey tries to reassure Dawson in the wake of her parents' divorce. Dawson is further moved by a father who worries over his injured son in the ambulance. When they're told that there's a fast track to permanently adopting Louie but it will be hindered by Dawson being seen as a single mother, they decide to marry with the whole team in attendance.

Meanwhile the ongoing battle to keep Molly's afloat comes to a head when Otis discovers 2016 is the one hundredth year the bar has been open and wants to throw an anniversary party. But the idea is nearly torpedoed by an overzealous Sun-Times reporter who decides to seize upon the bar having been the scene of a massacre. The bar is saved when her article gets it added as a stop on a local "crime tour."

And after yet another one-night stand Severide begins to reconsider his promiscuous lifestyle. When he goes to Chicago Med to be treated for an injury, Jeff Clarke asks him about donating bone marrow. Casey and Dawson's wedding prompts Severide to take Clarke up on the request.

But the episode ends with Louie's father showing up on the newlyweds' doorstep saying that he wants his son back.