It's a fairly normal day in the ED -- Sharon is trying to get Peter to go to the board and get the department a new scanner, it's Emily's first day volunteering, Will's still trying to get Nat to un-break up with him to no avail. All of a sudden, police cars pull up with shooting victims in the back. There's been a mass shooting in Millenium Park, and first responders can't get to the scene quickly enough. Sharon and Maggie get the ED running under protocol, with everybody with any medical know how pitching in. The doctors are initially overwhelmed, but Ethan sets up a new system where instead of doctors rotating between patients, each is assigned to a designated treatment center -- intubation, sutures, surgical assessment, etc. Still, resources are spread then and space is at a premium. The EMT lounge is turned into a temporary morgue and the doctors lounge into an ER. Latham is highly discomfited by the chaos of the situation and the disruption to his routine, but Connor is able to get him on the right path, even if he doesn't do so in the gentlest of ways. In the midst of the insanity, Natalie is worried about Owen and his nanny, who often spend time at the park. She can't get ahold of the nanny, and her worry just gets worse as more time passes, especially after a child is brought in on his own, having been trampled to death. Sarah is sent to oversee the retreival of personal items in the cafeteria, and to help the family members who might be distressed. Charles treats two patients who are in severe shock -- one man is in a stupor and is having trouble recalling events, and the other is suffering sever guilt and refuses to keep pressure on his neck wound, saying he deserves to die. The first man thinks he recognizes a man who is helping Emily bandage patients in the waiting room as the shooter, but Daniel realizes the shooter is actually the man with the neck wound when he over hears a comment from Jay about the gun that was used being traced back to a hoarders house. Just as he realizes this, the man has slipped out of the ED. Daniel rushes to the ambulance bay looking for him, and finds him in an alley, having slit his wrists and stabbed himself. An emotional Nat yells at Daniel for saving the shooter instead of innocent victims, but Sharon understands why he had to do so.