News of the incoming mass causalities after a gunman opens fire at a local college hits the staff hard when they must spring into action, even while their own wounds are still fresh; and Arizona does not receive a warm welcome home at the hospital.

Derek breaks ground on his Alzheimer's clinical trial and Meredith can't help but feel left out; Bailey's attempt to take the hospital into the age of new media with live tweets from the OR does not go over well with the Chief; and Callie, Mark and Arizona must learn to navigate through the pregnancy together, which starts off with Callie driving her new obstetrician crazy.