On the two-hour season premiere of "Grey's Anatomy," Meredith and Derek learn that "happily ever after" isn't easy, a military doctor who brings in a trauma patient catches Cristina's eye, and the Chief and his staff reel at the news that Seattle Grace is no longer nationally ranked as a top-tier teaching hospital.
Cristina's treated by Major Hunt after being impaled by an icicle and Meredith and Derek finally reach a conclusion about moving in together.
Plumbing leak becomes a deluge and wreaks havoc at Seattle Grace, as the Chief attempts to implement new teaching policies and George tries to retake his residency exam; meanwhile Derek hopes to move the roommates out of Meredith's house, to their surprise.
Meredith freaks out when Derek discovers her mother's old diary in the house, Callie frets as she prepares for her first official date with Erica, and Cristina stumbles into a part of the hospital that neither she nor her fellow residents have seen before.
Bailey heads up a team of surgeons performing a "domino procedure" in which each surgery hinges on the one preceding it; Derek gets sole credit for the brain tumor study he did with Meredith; Lexie goes to great lengths to have George notice her romantically, and Sloan counsels a desperate Callie on the ways of love.
To help Bailey become the best general surgeon at Seattle Grace, the Chief gives her new power, new responsibility, and the challenge of removing a young girl's inoperable tumor, as the new head of trauma, Owen Hunt, challenges both the attending doctor's methods and the residents' surgical abilities.
The Chief announces to the residents that a solo surgery will be awarded, and the news sends them into a competitive frenzy. Meanwhile Derek turns to Bailey for advice when he feels Cristina is interfering in his relationship with Meredith, and Erica discovers a shocking connection between one of her surgical residents and a longtime patient.
One of Meredith's oldest friends, Sadie, becomes an intern at Seattle Grace, but Mer's friends are less-than-welcoming to her; Lexie leads her fellow interns in secretive, unorthodox surgical training sessions; and a new surgeon, Virginia Dixon, brings rules and order to the OR.
Meredith, Cristina and Bailey come to Lexie and Sadie's rescue when a routine surgery goes horribly wrong, as Owen and Derek treat a man seriously injured while sleepwalking and Mark comforts the sleepwalker's distraught daughter.
Cristina is awarded the first solo surgery among the residents, but must give it up and choose her replacement. Meanwhile Mark performs a cutting edge procedure to restore a patient's ability to speak -- and impresses Lexie in the process -- and Callie can't figure out if Sadie is just friendly or flirtatious.
Bailey teams with Seattle Grace's new pediatric surgeon, Dr. Arizona Robbins, to save the life of a young patient near and dear to her heart, as the admittance of a death row inmate at the hospital stirs anger in Derek and further erodes Meredith and Cristina's friendship; meanwhile, Callie and Sloan work as a team to fight their respective attractions to Sadie and Lexie, and Izzie tells Alex that she's seeing Denny.
Derek's mother, Carolyn, makes a surprise visit to Seattle and meets Meredith for the first time, as Mark tries to conceal his relationship with Lexie from her as well; meanwhile, death row inmate William wishes to donate his organs to Bailey and Arizona's patient, as Owen and Cristina try to go on their first date.
Bailey grows desperate as a patient's condition becomes more and more dire. Everyone in the hospital is stepping up to try to find the boy a new organ donor, including Meredith, who considers a death row patient's wish to die before execution. Meanwhile, an injury leaves Mark in a compromising position in the OR and Cristina awkwardly avoids Owen.
Now that Derek has the engagement ring, he searches for the perfect way to propose to Meredith, as Lexie demands that Mark go public with their relationship, and Bailey, encouraged by her work on recent cases and by Dr. Dixon, considers a new direction for her medical career.
Addison, Naomi and Sam rush the ailing Archer to Seattle Grace and reunite with old friends Derek and Mark to help save his life; meanwhile, Izzie employs her fellow doctors in a medical game designed to help teach the interns, and Owen runs into a significant someone from his past.
Derek's confidence is shaken like never before as he faces off with Addison over what course is best for their dying pregnant patient; Cristina calls the abilities of a renown surgeon into question after she botches a standard procedure; and Bailey, who has decided to pursue a fellowship in Pediatrics, is appalled by the Chief's less-than-stellar recommendation letter.
After learning at a deposition that more of his patients have died than survived, Derek decides to quit, even as Meredith refuses to give up on him. Meanwhile Bailey and the Chief continue to bicker over her peds fellowship -- until Adele steps in to mediate -- and Izzie's interns discover something unsettling on a "patient x," unaware that the charts they're looking at are actually Izzie's.
After his fellow surgeons try and fail to bring Derek back to work, Meredith must summon the strength to convince him. Meanwhile the hospital is abuzz as Mark attempts a rare face transplant surgery, Cristina, at last awarded her first solo surgery, is more focused on the needs of her friends, and Izzie does her best to keep her medical condition a secret.
Tensions are running high at Seattle Grace with all eyes on Derek as he is set to perform his first surgery after his breakdown; attention is also set on Izzie, whose medical condition requires her to undergo a difficult surgery; and Owen has another PTSD episode that injures Cristina and traumatizes both of them, throwing the future of their relationship into question.
As she endures aggressive treatment for her cancer, Izzie throws herself into planning Derek and Meredith's wedding, Callie introduces Arizona as her girlfriend to her disapproving father, George excels in a trauma emergency when Alex freezes, and Owen seeks help with his PTSD.
As preparations continue for Meredith and Derek's wedding, Izzie receives a surprise hospital visit from her handful-of-a-mother, Robbie. Meanwhile, a newly sober Thatcher visits his daughters, Meredith and Lexie, in order to make amends, and the Chief and Meredith bitterly fight over a controversial medical case.
As Meredith, Derek and all of their friends prepare for the big wedding, a group of college students, injured in a car crash as they were heading to their graduation ceremony, is admitted to the ER. Meanwhile, Izzie fears the worst when she begins experiencing familiar symptoms.
A plumbing leak becomes a deluge and wreaks havoc at Seattle Grace, as the Chief attempts to implement new teaching policies.
George delivers stunning news to Bailey, sending shockwaves throughout the hospital, and Izzie's friends anxiously await her recovery from surgery. Meanwhile Bailey is surprisingly displeased after being accepted into the pediatric fellowship program, and the victim of a near-fatal traffic accident brings the talents of Seattle Grace's doctors together.