In the two hour fifth-season opener, Meredith and Derek discover the morning-after side of their renewed relationship; a military doctor (Kevin McKidd) who arrives at the hospital with a trauma patient catches Cristina's eye; and the chief learns that Seattle Grace's national rankings have slipped..

While Meredith and Derek continue in their new relationship, Cristina continues to get to know the military doctor as they treat his trauma patient.

A 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 make some changes at Meredith's house.Derek attempts to get Merediths roommates to move out, much to their surprise.

Meredith is sent into a state of panic when Derek finds her mother's diary in the house. Meanwhile, Callie stresses while preparing for her first date with Erica. Later, Cristina stumbles across a part of the hospital no one has ever seen before.

Bailey leads a team for a domino procedure in which each surgery is dependent on the one before it. Meanwhile, Derek receives sole credit for the study he and Meredith worked on; Lexie tries to get George to see her in a different light; and Sloan gives Callie relationship advice.

The Chief gives Bailey new responsibility in order to help her to be one of the best surgeons at Seattle Grace. Meanwhile, Owen Hunt, the new head of trauma, clashes with the current staff's methods and abilities.

episode 7..The Chief tells the residents that one of them will get a solo surgery, and a competitive frenzy ensues; Derek asks Bailey for advice when he feels that Cristina is coming between him and Meredith; Erica discovers a shocking connection between one of her residents and a longtime patient. .

episode 8.Meredith's old friend Sadie (Melissa George) becomes an intern at Seattle Grace, but Meredith's other friends aren't exactly welcoming; new surgeon Virginia Dixon (Mary McDonnell) brings rules and order to the OR; and Lexie leads the interns in secretive, unorthodox surgical-training sessions. .

Meredith, Cristina and Bailey must save Lexie and Sadie after a routine surgery goes terribly awry. Meanwhile, Owen and Derek treat a man injured while sleepwalking.

Cristina wins the first solo surgery among the residents, but is forced to give it up and choose who will perform the surgery in her place. Meanwhile, Mark heads up a procedure that will allow his patient to regain their ability to speak. Elsewhere, Callie tries to figure out if Sadie is being friendly to her or hitting on her.

Bailey pairs up with Dr. Arizona Robbins, Seattle Grace's new pediatric surgeon, to help save the life of a young patient. Derek is furious when a death row inmate is checked in to the hospital; Cristina and Meredith's friendship continues to erode; Callie and Sloan work through their attractions to Sadie and Lexie; and Izzie confesses to Alex that she has been 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.

With the engagement ring taken care of, Derek ponders the perfect way to pop the question to Meredith. Meanwhile, Lexie demands that Mark go public with their relationship; and Bailey, happy with her work with recent patients, considers a new career direction.

Addison, Naomi and Sam bring Archer to Seattle Grace in order to have Derek and Mark work to save his life. Meanwhile, Izzie works on a medical game to help the interns.

Derek's confidence is dashed as he squares off with Addison over a patient. Cristina begins to doubt the abilities of a surgeon after she messes up on a routine procedure. Meanwhile, Bailey, pursuing a fellowship in pediatrics, is appalled by the Chief's recommendation letter.

When Derek learns that more of his patients have died than survived, he decides to quit despite Meredith standing by him. Meanwhile, Bailey and the Chief continue to fail to see eye-to-eye on the fellowship until Adele steps in. Elsewhere, Izzie’s interns discover dire news in regards to patient x, unaware it’s Izzie.

Stand By Me 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 high as Derek returns to Seattle Grace to perform his first surgery following his breakdown. Meanwhile, everyone turns their attention to Izzie, who must undergo surgery for her condition; and Owen suffers from another PTSD episode that injures Cristina, putting their relationship at risk.

As she endures aggressive treatment for her cancer, Izzie throws herself into planning Derek and Meredith's wedding.

As preparations continue for Meredith and Derek's wedding, Izzie receives a surprise hospital visit from her handful-of-a-mother, Robbie.

As Meredith, Derek and their friends and family gather for their wedding, a group of several college students are admitted to the E.R. following a car crash.

After spending time with a fellow cancer patient, Izzie begins to question whether or not to undergo a risky procedure. Meanwhile, Mark considers taking his relationship with Lexie to the next level; and after helping a patient, Owen begins to rethink his position at Seattle Grace.

George gives Bailey news that shocks the entire hospital as everyone awaits for Izzie to recover from surgery. Meanwhile, Bailey is accepted into the pediatric fellowship program; and Seattle Grace comes together to help save a traffic accident victim.