In the season premiere, the hospital staff is trying to deal with physical and emotional trauma in the wake of the deadly rampage of a vengeful gunman. As Dr. Perkins, a trauma counselor, is brought in to help in the recovery and to assess each doctor's readiness to return to work, Derek makes a spontaneous decision to resign as Chief and rushes back into surgery, taking everything a little too fast, and Cristina buries herself in her wedding planning.

When Derek receives an unexpected and unwelcome visit from his estranged sister, Amelia, issues between the siblings -- both past and present -- come to the surface; the Chief tries to help Alex when he notices that he's refusing to use the elevators after his near-fatal shooting; and Meredith and Derek continue their efforts to ease Cristina back into surgery after her post-traumatic stress.