Boden learns his firehouse is in danger of getting shut down. Severide is being targeted by an arsonist. Mouch brings his new girlfriend to the firehouse. Molly's is facing a competition from a new sports bar down the street.

The threat of an arsonist with a vendetta against the firehouse looms and Casey struggles with the repercussions of Heather Darden's accident.

Severide is convinced he knows the arsonist's identity.

Internal issues threaten to derail Engine Company 51.

The future is uncertain for more than just Boden and Firehouse 51.

McLeod continues to pressure Chief Boden, threatening to close Station 51, since he failed to retire as planned. "Mouch" steps up to pressure the union leader to preserve the station.

An explosion at a hospital where Casey and Dawson are volunteering for a charity run sends Chicago into chaos.

A fire at a boarding school shakes up Lt. Severide; Lt. Casey tries to help Dawson; Mills looks into his father's past.