The firehouse is on high alert when mysterious threats begin to emerge throughout Chicago. The situation turns dire when news of a mass shooting brings members of squad and truck into the middle of a dangerous and harrowing scene. Lt. Casey works diligently to investigate the missing money from the fundraiser for the victims of the recent tornado. Meanwhile, Jimmy agrees to represent the firehouse in the first annual "Battle of the Badges" boxing match against the 21st district police department and Chili's erratic behavior continues to be a major issue for her ambulance partner, Brett.