The show explores the lives, both professional and personal, of the firefighters and paramedics of the Chicago Fire Department at the quarters of Engine Co. 51; Truck Co. 81; Rescue Squad Co. 3; and Ambulance 61. Following the death of veteran Firefighter Andrew Darden, loyalties fracture and divide as Lieutenant Matthew Casey, officer-in-charge of Truck Co. 81, and Lieutenant Kelly Severide, officer-in-charge of Squad Co. 3, both blame one another for the death of their long-time friend and colleague.