Lt. Severide lobbies Chief Boden for a favor for Scott Rice, an old friend needing a second chance with the CFD, but Casey and Otis have their doubts. Elsewhere, Mills works to regain his active squad status while Brett and Dawson decide they need a big night out.

It's all hands on deck when Firehouse 51 races to the scene of a deadly chemical leak inside an industrial building. With squad and truck tested like never before, scores of workers are rescued but many are experiencing respiratory distress. This prompts truck 81, squad 3 and ambulance 61 to spring into action to transport the victims to nearby Chicago Med. The situation takes a turn from bad to worse in the midst of the chaos when a "patient" planted inside the hospital goes on a suicide mission armed with a grenade and claiming to be the carrier of a deadly airborne disease. Trying to stop the madman from engaging the explosive device, a member of the firehouse team is severely wounded in the blast and left to face life-threatening injuries. With no other choice, Chicago Med is forced into lockdown until doctors and the Hazmat team can determine the severity of the airborne virus.