Top security measures go into place as a team of doctors, led by the Chief, try to save the life of a major political figure from the Middle East. Meanwhile Cristina decides to finally decorate her new place and throw a housewarming party, and Alex shows up late from a weekend getaway to his first day of rounds with the new Pediatrics Attending, Dr. Robert Stark.

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.

News of Meredith and Derek's unsteady relationship raises a red flag for Zola's adoption counselor; Alex quickly realizes that he has become the outcast of the group after ratting out Meredith; and Cristina makes a tough decision regarding her unexpected pregnancy. Also, Chief Webber brings Henry in for a last minute surgery, alarming Teddy.

Jackson's suspicions about Mark and Lexie interfere with work.

Meredith dreams of what her life would have been.

Derek encourages a hesitant Meredith to work with him again in neurology. When she complies, she listens as Derek is talking to a patient, who excitedly tells him about how she passed out in the middle of a big job interview. They're going to remove a cyst that's causing seizures. There is also a benign tumor that they aren't going to remove. Just as the procedure is past the opening skull stage, Derek is called to another OR where Morgan's baby is suffering from a brain bleed. He tells Lexi and Meredith to carry on. A slight bleed leads to another chunk of skull being removed. This opens up sight of the benign tumor. They decide to remove it because access is already present. Derek is incensed when he returns. It wasn't in the plan. When the patient awakes, she is unable to speak. Her parents threaten to sue. Jackson's mother, Catherine Avery, returns to Seattle Grace with a urology fellow, to replace a faulty bladder with one that Richard and Miranda have grown in the lab. During the procedure, Catherine develops an interest in Richard, becoming quite flirtatious. The fellow, Dr. Mara Keaton, was brought to be a spy for Catherine about how her son is doing. Jackson is forced to take her on a hospital tour. He is instantly suspicious of her motives and calls her on it. She admits that she is supposed to gather information on him. In turn, he advises Mara on how she can assert herself and gain his mother's respect. They try it out, defying her intent to take them to an awards banquet. The two of them trip off to find an empty on call room to get to know each other better. Richard is asked to the award banquet but turns her down at the last minute. Cristina becomes increasingly suspicious of Owen's every move. She finally corners nurse Emily, her prime suspect, and asks her point blank about sleeping with Owen. Emily says no. She uses a bit of flirting to get through long shifts. When Christina faces Owen, he admits he has been unfaithful to her. Alex has been working continuously to save Morgan's preemie. Callie watches Alex and Morgan work together and sees that Morgan is falling for him. He denies it. But when Morgan has her picture taken with him and the baby, you can see it in her body language.

Cristina's return to Seattle proves to be awkward when she and Owen must face each other again. Meanwhile, Jackson and April continue to try and negotiate their relationship, Jo deals with a perplexing case, and Bailey attempts to get Arizona into the hospital. Then, the survivors of the crash are shocked with new information about the cause of the crash.

It's Arizona's first day back at work and Callie tries to keep things as normal as possible for her. Meanwhile, Bailey argues with Ben over wedding plans and Cristina and Owen continue to struggle with their relationship while Owen faces his first set of meetings about the plane crash.

As Bailey puts her wedding on hold to tend to Adele, a biker gang floods the hospital with injuries for all the other doctors to tend to. Meanwhile, Cristina and Owen make a final decision about the divorce and Callie and Arizona try to reconnect.

As the hospital inches closer to finding a buyer, many of the doctors don't like the new changes being made, causing the crash survivors to consider a crazy idea. Meanwhile, April's relationship with Matthew progresses and she asks Jackson for advice.

The transition to new management has the doctors at each others throats. Matters become worse when Owen quits, leaving the OR board in complete disarray.

With a major storm rolling in, the hospital is chaotic. When Jason comes into the hospital in critical condition, Alex does everything he can to protect Jo. With Meredith's due date just weeks away, Derek urges her to slow down.

Costumed patients create a challenge for the doctors; Derek and Ben work on a new surgical technique; Richard asks a resident to perform his next surgery, angering Bailey.

Meredith and Cristina bicker about their careers; Bailey talks to Ben about him moving back to Seattle; Shane puts himself in a risky situation; Derek receives a life-changing phone call.

Arizona's bad day escalates when she advises a patient on a potential double amputation; Derek breaks a promise; Stephanie and Jo find an abandoned baby; the nonfraternization rule leads to a suspension.

The doctors of Grey Sloan Memorial Hospital succumb to an infection that spreads like wildfire. Cristina deals with a complex case involving siblings and Alex begins to consider his future in the hospital.

Callie and Arizona discuss having another baby, Derek has a hard time juggling work and family, Jackson and April have their first big fight and Christina attends the Harper Avery Award Ceremony. But is she going to win?

Cristina must head to Zurich soon but she is unprepared to leave behind her life in Seattle. Fear runs amok throughout the hospital as an explosion in a shopping mall results in a mass casualty situation.

Arizona has to take care of one of Herman's patients. Everybody finds out Amelia was a drug addict and Derek lets her down. Jo has her first solo surgery. Maggie is invited to Ellis' house and Arizona decides to move in with Alex.