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.

A gunman opens fire at a local college, sending mass casualties to the hospital; Arizona's return is not warmly received.

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.

This is an historic episode that finally crosses a long-standing barrier.

The doctors become nostalgic when George O'Malley's mother, Louise, returns to Seattle Grace for medical help after a botched surgery at a neighboring hospital; the residents are inspired by a medical miracle when they witness a harvested heart that continues to beat outside the body; a new pediatric fellow excites Arizona and makes Alex feel threatened; Jackson lets his suspicions about Mark and Lexie interfere with his work; and Henry and Teddy have their first marital fight when he expresses interest in pursuing medical school

As Meredith puts Zola to bed and falls asleep, she begins to wonder -- what if her mother had never had Alzheimer's and she'd had loving, supportive parents? The reverberations of a happy Meredith Grey change the world of Seattle Grace as we know it. What if she had never met Derek in that bar and he had never separated from Addison? What if Callie and Owen had become a couple long before she met Arizona? And what if Bailey never evolved from the meek intern she once was?

Derek encourages a hesitant Meredith to work with him again in neurology; Jackson's mother, Catherine Avery, returns to Seattle Grace with a urology fellow and develops an interest in Richard; and Cristina becomes increasingly suspicious of Owen's every move. Meanwhile, Alex tries to deny the fact that Morgan may be falling for him.

As the doctors continue with their lawsuit, they’re forced to confront the realities of their injuries. Meanwhile, Bailey tricks Arizona into helping her with a pediatric case, and Cristina tries to find normalcy in her new environment.

Cristina and Owen continue to figure out their relationship amidst the pending lawsuit; and Bailey argues with Ben over plans for their upcoming nuptials. Meanwhile, Callie recruits Jackson to help find a solution to Derek's hand, but he is preoccupied by a shocking situation with April.

Bailey puts her wedding day on hold and continues her efforts to save Adele's life, Cristina and Owen remain undecided about their pending divorce, while Arizona and Callie try to get their spark back. Meanwhile, the hospital becomes inundated with a group of bikers after a horrible accident.

As the hospital spirals closer to financial ruin, Owen must make some tough decisions, which includes foregoing an expensive surgery that could save the life of a child. Meanwhile, Alex and Jo work together to save the life of an infant, and April asks Jackson for dating advice.

The discord and chaos created by the hospital's new management team causes one physician to question his future at Seattle Grace. Meanwhile, the doctors and the interns must juggle several transplant surgeries at once, each coming with its own set of obstacles.

The doctors of Grey Sloan Memorial Hospital prepare for a super storm heading towards Seattle. Owen's desire to be a parent gets stronger as he becomes closer to Ethan. One wrong move puts the lives of Meredith and her baby in jeopardy. Meanwhile, Jo looks to Alex for help in a moment of crisis, and one doctor gets the shock of her life.

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.

On the day of April's wedding, the Grey Sloan Memorial doctors are all preoccupied with their own drama. Meredith and Cristina continue to argue about their research and careers, Bailey comes clean to Ben about her feelings regarding him moving back to Seattle, and Shane puts himself in a risky situation. Meanwhile, Derek gets a phone call that will change his life.

Arizona has a bad day which escalates when she advises one of Callie's patients on a potential double amputation. Derek goes back on a promise to Callie, and Jo and Stephanie come across an abandoned baby outside the hospital. Meanwhile, the doctors deal with the repercussions of the non-fraternization rule, which include meetings with HR and one doctor's suspension.

The hospital gets inundated with flu infected patients, which causes the doctors to slowly fall ill one by one. Derek must fight off the sickness as he prepares to give a speech on brain mapping and the residents place bets on who will fall ill first. Meanwhile, a new doctor at the hospital gives Alex a glimpse into an alternate form of medical practice.

Cristina enlists Meredith to help her prepare an acceptance speech for her possible Harper Avery win and while Owen offers to attend the ceremony with her, she insists on going alone. Meanwhile, Derek feels overwhelmed between his responsibilities at home and at the hospital. April and Jackson come to a disagreement on children and religion, and Callie and Arizona try to reconnect in their marriage.

The doctors of Grey Sloan Memorial Hospital bid an emotional farewell to the beloved Cristina Yang. A possible act of terrorism rocks Seattle and causes chaos at the hospital. Meanwhile, Meredith makes a life changing decision and one doctor receives shocking news.

Information about Amelia's past emerges and causes problems for her at the hospital.