The staff tries to deal with physical and emotional trauma in the wake of the vengeful gunman's deadly rampage; Derek makes a spontaneous decision to resign as chief and return to surgery.
Meredith finally tells Derek about her pregnancy and miscarriage; during a surgery, Cristina experiences PTSD; Bailey won't let Alex operate until he has the bullet in his chest removed.
Issues surface when Derek's estranged sister, Amelia (Caterina Scorsone), visits unexpectedly; Meredith and Derek continue to try to ease Cristina back into surgery.
Derek and Meredith visit the obstetrician and receive disturbing news; Lexie notices April and Meredith growing closer; Jackson tries to get ahead in the operating room.
As part of their training, Seattle Grace's residents are put in charge; the attending surgeons compete for a million-dollar grant.
A documentary crew visits the hospital six months after the shooting to document the road to recovery for doctors and patients; Meredith helps Cristina put on a brave face for the cameras.
Owen puts the residents through a rigorous trauma certification drill; Cristina is left alone to monitor an ill lung transplant patient.
A team of doctors tries to save the life of a political figure from the Middle East; Cristina decides to decorate her new place and throw a housewarming party; Alex shows up late to his first day of rounds with the new pediatrics attending.
Derek and the attendings go out to celebrate when he gets the grant he needs for his clinical trial; Meredith and Alex must fend for themselves during the night shift.
Concerned about Cristina, Derek takes her on a fishing trip; Teddy directs her efforts and compassion toward a dying patient (Scott Foley) without the proper insurance to seek treatment.
A gunman opens fire at a local college, sending mass casualties to the hospital; Arizona's return is not warmly received.
Arizona tries to reconcile with Callie, who has big news to share; Teddy faces an important decision; the residents receive new interns; the Chief makes an announcement.
Meredith feels left out when Derek breaks ground on his clinical trial; Bailey's new idea does not go over well with the Chief; Callie irritates her new obstetrician (Rachael Taylor).
Meredith and Lexie's father returns to Seattle Grace with stomach pains and a new girlfriend; Alex butts heads with the new OB; Mark bribes Jackson for information on Lexie.
Meredith volunteers to run the ER for a night in hope of proving that she is chief-resident material; Bailey and Eli (Daniel Sunjata) get into mischief; Adele comes to the hospital as a patient.
Meredith's fertility treatments start affecting her eyesight; Mark talks to Callie and Arizona about his role as a father; April learns more about Dr. Stark (Peter MacNicol).
Richard tries to get Adele into Derek and Meredith's Alzheimer's trial; Arizona and Mark try to plan a baby shower for Callie; Henry has another medical complication; competition for the chief resident position is amplified.
A cataclysmic event leaves the doctors shaken; Callie, on the verge of a life-changing moment, starts imagining about the hospital and her friends.
Everyone tries to help Callie and her baby recover; the sudden death of a clinical trial patient impacts Derek and Meredith in different ways; Teddy is surprised by a familiar face.
Callie and Arizona's wedding is not coming together as they hoped it would; Alex seems to be the first choice for chief resident; Meredith and Derek make a life-changing decision; Dr. Perkins tempts Teddy with a proposition.
Work and personal pressures leave Meredith on edge; formal interviews for the chief resident position begin; Lucy and Alex's relationship is tested.