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.
Casey returns to work after a brush with death; Shay's new partner is friendly to everyone but her; Dawson begins training; Severide prepares for his instructor duties; Clarke refuses to talk to the police about the murder.
Casey tries to convince everyone he is healthy; Shay avoids the lawyer who wants to talk about Daryl's suicide; Otis and Katie grow closer; Boden helps a woman with a negligent landlord.
A drunken driver takes out a transformer, leaving the entire neighborhood without power on a frigid night; Casey continues to lie about his condition.
Severide must leave Lindsay and the Intelligence Unit to find Katie; Blackhawk players Brent Seabrook and Duncan Keith give Shay a tip; Casey gets more information about his head injuries; Dawson works to pass the physical test.
Severide becomes frustrated with the police; a girl is trapped in a donation bin; a fire breaks out in a 20-story building downtown.
Detective Lindsay calls in suspects in the disappearance of Katie's kidnapper; Jones struggles to fit in; Herrmann celebrates his 20th wedding anniversary.
Severide feels compelled to help a man he saved from a car accident; Casey and Boden struggle to deal with the orders from Jones' father; a call to duty affects the firehouse.
Lt. Severide tries to help a firefighter from Denver struggling with addiction; Shay and Dawson plan a girls weekend at a cabin in the woods; Mills and the guys update Mouch's online dating profile.
Surprised Dawson receives a suicide note; Severide looks into Bloom's dark past; Shay and Casey search for a life-changing item.
A large explosion sends Chicago into chaos; the members of the firehouse are tested when one of their own goes missing in the blast.
A call to the field may throw a wrench in Dawson's plan to retake the firefighter exam; the firehouse welcomes a new addition; Boden makes a bold move.