Lt. Severide is targeted by an arsonist; Battalion Chief Boden clashes with Gail McLeod; the anniversary of Andy's death; Shay has suspicions; Mari visits Mouch.

Casey struggles with his new responsibilities; Severide dreads the arsonist's next attack; Mouch starts campaigning; Dawson and Shay decide they need to get out more.

Severide tries to prove his suspicions about the arsonist's identity; Casey gets accustomed to his new lifestyle; Dawson goes on a date with a regular from the bar.

Severide wants to implicate the serial arsonist; Dawson and Shay face the unexpected on a routine call; suspicions indicate Clarke is the mole; Mouch campaigns for union president.

McLeod pushes Boden toward an early retirement and chooses a potential replacement; all make known their disdain for the mole; Shay enjoys a full social life.

Severide comes across a boy in need of help while out on a jog; Boden and McLeod clash over the future of the firehouse; Mouch and Greg Sullivan debate on Otis' podcast.

The firehouse crew goes to the scene of a railroad accident; Boden must make a big decision about his future; Severide learns more about the woman he saw with his father; Shay and Dawson continue to butt heads.

The shooting of a child in their neighborhood forces Cruz and his brother to think about the gang; Isabella gets Mills to dress up for a gala; Casey feels sad he will be seeing Dawson less; Shay gets in deeper with Devon.

Boden tries to keep the firehouse from closing; the romance between Dawson and Casey grows; Cruz worries for his brother's safety; Severide and Katie make up for lost time.

Chief Boden leads the fight to keep the firehouse open; an emergency call to an apartment structure puts the squad in harm's way; Dawson receives life-changing news.