Firehouse 51 is forced to share space with Firehouse 17; Kidd and Herrmann fight over Ritter; an arson case tests Kidd and Severide's relationship; Firehouse 51 says goodbye to one of its own.
Boden pushes his team to pull it together in the aftermath of the explosion, Casey works to track down a missing Severide, Newhouse brings news about one of Mills' relatives and Herrmann and Otis come up with new goals.
Mouch sets out to change his legacy and nickname; Ritter pushes Herrmann to see a doctor; Brett searches for the perfect wedding venue.
Severide wants to implicate the serial arsonist, Dawson and Shay face the unexpected on a routine call, suspicions indicate Clarke is the mole, and Mouch campaigns for union president.
Casey finds himself in dangerous territory when working undercover to bring down a trafficking ring. Also, Dawson realises that her life is about to change, and a new candidate makes an unusual first impression.