Lt. Severide teams up with Seager and the OFI when a motel structure fire turns suspicious. Firehouse 51 members become amateur sleuths following a Reddit post involving one of their own. Brett continues to get invested in her birth mother, Julie.
- Jesse Spencer as Lieutenant/Captain Matthew Casey
- Taylor Kinney as Lieutenant Kelly Severide
- Kara Killmer as Paramedic Sylvie Brett
- David Eigenberg as Firefighter Christopher Herrmann
- Joe Minoso as Firefightern Joe Cruz
- Christian Stolte as Firefighter Randy "Mouch" McHolland
- Miranda Rae Mayo as Firefighter Stella Kidd
- Eamonn Walker as Battalion Chief Wallace Boden
- Annie Ilonzeh as Paramedic Emily Foster
- Julie dies giving birth.