Entering their first year as residents, Meredith Grey, Cristina Yang, Izzie Stevens and Alex Karev now have interns of their own to mentor. Among the new interns are George, forced to repeat his internship after failing his exams, and Lexie Grey, Meredith's half-sister. Another two new interns will be introduced in this season premiere. And they'll have their hands full when a three-car accident sends multiple casualties pouring into Seattle Grace. Some of the doctors need to deal with new stressed out interns. A young boy brings his bleeding deer to the hospital in the hope a doctor will save her.

A patient is in the hospital because they fell asleep in a hazardous situation. A patient's home laboratory blew up in an apartment building and the victims flood the hospital. Alex investigates the cause of the incident by talking to a really old guy who lived in the building. Jane Burke shows up to deal with what her son left behind. Lexie tries to talk to Meredith. Callie is overwhelmed by her new position while Bailey is searching for things to do.

The doctors meet the world's oldest intern. Marcy brings in her son Henry because she finds his change in behavior suspicious. A patient named Connie may lose her vocal chords unless Mark and Richard can perform a radical new surgery. George impresses the other interns at work but at home he is trying to deal with his guilt about having sex with Izzie.

A face familiar to one of the year-twos shows up and dredges up issues that were never really dealt with. A teen soccer player needs counseling because of his father and surgery for an injury he sustained while playing the game. Another patient's, named Courtney, finds out that her cancer is back.

It's Halloween at Seattle Grace! There may be jokes, pranks, and/or scare tactics. Some of the women form a club based on an experience they all share. A man volunteers for what most of the doctors believe is an unnecessary surgery. One of the patients is in the hospital because they tried to carve a pumpkin in an unconventional manner.

A patient comes to the ER with a dream and a condition which may prevent him form realizing it. Two patients are competing to win something big. One of them convinces an intern to take their place.

Izzie and George's new-founded relationship results in them having bad sex. Callie's Chief Resident position is taken away from her, and Bailey is the humble replacement. Meredith can barely handle it with Lexie anymore.

There has been a bus crash and the patients are in the ER. Meredith and Derek's relationship gets more complicated when he dates a fellow doctor. Meredith's dad goes to visit her at the hospital.

An ambulance crash endangers the lives of the paramedics involved, as Meredith and the Chief work on-site to save them, Bailey treats a patient who refuses her help, and Ava returns for Alex and meets his other girlfriend in the process.

Meredith works furiously to save a paramedic trapped in an ambulance; Lexie holds the life of a patient in her hands; Derek must rely on a nurse to save a patient; Bailey struggles to balance her personal life with her work.

George has a surprise visitor. Bailey's family is falling apart. Derek sorts through what kissing Rose means. Meredith discovers something that doesn't inspire hope or happiness in her. Alex treats a faith healer.

Time has passed but not much has changed. Sexual angst still runs rampant in the halls of Seattle Grace. Derek and Rose have been dating and Meredith has noticed the activities of her current off-again ex-boyfriend. The doctor's are in the midst of a competition.

Addison visits Seattle Grace because they need skills that only she has. Rebecca/Ava returns with news for Alex. Meredith and Derek's clinical trial experiences one or two developments.

The clinical trial continues to make progress. The anti-McSteamy club creates new initiatives but that isn't enough to stop one woman from spending time with him. Christina is affected by Burke's career developments.

Dr. Wyatt refuses to let Meredith give up on therapy. Alex learns something else about Rebecca. The Chief wants Erica to perform a daring surgery. Christina continues to act strangely.

A couple undergoes surgery. Derek and Meredith's clinical trial gets a second shot from a new patient. Izzie helps Alex. McSteamy hooks up with someone.

A couple undergoes surgery. Derek and Meredith's clinical trial gets a second shot from a new patient. Izzie helps Alex. McSteamy hooks up with someone.

As Meredith and Derek begin to find that their "happily ever after" is anything but easy, Cristina develops a crush on a military doctor who brings in a trauma patient. Meanwhile, the Chief and his staff cope with the news that Seattle Grace is no longer considered a top-tier teaching hospital.

Alex is offended when Izzie talks about how great a man Alex is to Meredith. Derek is stabbed with the scalpel in the palm of his hand by Rose, while the Chief is stunned over Mercy West's increase in the number of patients, a deadly ice storm ravages Seattle.

When the plumbing springs a leak, chaos breaks out at Seattle Grace as the Chief tries to put in place new teaching methods. Meanwhile, George takes his residency exam; and Derek attempts to get Meredith's 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 heads up a "domino procedure" in which each surgery hinges on the one preceding it. Meanwhile, Derek gets sole credit for the brain-tumor study he did with Meredith; Lexie tries to get George to notice her romantically; and Sloan counsels a desperate Callie in the ways of love.

The chief gives Bailey new power and responsibility. He also gives her a very difficult surgical assignment: remove a girl's inoperable tumor. Meanwhile, new trauma chief Owen (Kevin McKidd) challenges the attendings' methods and the residents' skills.

When the Chief makes an announcement that a solo surgery is up as an award, the residents are sent into a frenzy all trying to get an edge over one another. Meanwhile, Derek asks Bailey for advice after Cristina continues to interfere in his relationship with Meredith. Elsewhere, Erica uncovers a shocking connection between a resident and a patient.

One of Meredith's oldest friends, Sadie, becomes an intern at Seattle Grace, but Mer's friends are less-than-welcoming to her.

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 she is forced to give it up and choose her replacement; Lexie is impressed when Mark performs a cutting-edge procedure; Callie doesn't know if Sadie is 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.

Derek decides to quit after learning that more of his patients have died than survived; Izzie's interns make an unsettling discovery about "Patient X."

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.

All eyes are 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.

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.

In the first part of the two hour season finale, Izzie spends time with a fellow cancer patient and contemplates whether or not to undergo a risky surgery, as Mark is ready to take his relationship with Lexie to the next level and a patient makes Owen reassess his place at Seattle Grace.

In the second part of the two hour season finale, 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. This is the second hour of a 2-hour season finale.

The hospital staff is left to deal with the aftermath of George's passing. Hitting the staff hard, they all find unique ways to get through the various stages of grief. George's mom returns, faced with the difficult task of deciding what to do with his organs.

Everyone is struggling after losing one of their own and are turning to any distraction they can find. Meredith and Derek seem to be consummating their marriage anywhere and everywhere, while Cristina and Owen have been instructed to abstain from sex by Dr. Wyatt.

Several weeks after her near-death experience, a recovering Izzie returns to work. Meanwhile, nervous that there will be layoffs in the hospital in the wake of an announcement from the Chief, the Seattle Grace residents compete to survive the cuts, including Cristina, who attempts to work alongside Arizona in pediatrics. And Bailey and Alex's patient's already complex case is further complicated by her schizophrenic son.

When Meredith and Lexie's father, Thatcher, returns to the hospital with a failed liver, it's up to Meredith to save his life. Meanwhile, Izzie empathizes with a patient riddled with tumors, as Mark, annoyed with Cristina's competitive zeal, tricks her into assisting on an unusual surgery.

The residents of Seattle Grace and Mercy West vie for surgeries and their careers as day one of the hospital merger begins. Meanwhile, Arizona stands by an enraged Callie when her father, Mr. Torres, employs a priest to help convince her to date men, and Izzie experiences the brunt of her new surgical competition.

After a burn victim unexpectedly dies in the midst of a chaotic ER, the Chief and board member Jennings interrogate the residents to find the culprit, as all the Mercy West and Seattle Grace resident defends their actions in an effort to save their jobs.

When Isaac has an inoperable tumor wrapped around his spine, he wants Derek to do the impossible. Derek tests the Chief's authority when Richard objects to moving forward with the risky surgery.

A generous 25 million donation is offered to Seattle Grace by the parents of Arizonas 10-year-old patient, she finds herself in a conflict of interest when his condition worsens. Cristina tests her boundaries with Owen at the hospital. Alex is left with all of Izzies hospital bills to contend with.

Owen recruits Dr. Teddy Altman, a former Iraqi veteran, as the new cardiothoracic surgeon. Cristina questions her abilities in the E.R. and relationship with Owen. Izzie returns to Seattle Grace with her high school mentor seeking treatment for his dementia. Richard's responsibilities, post-merger, are taking a toll on him.

As Thanksgiving, Christmas and New Year's pass, Miranda gets a visit from her father, William who disapproves of her choices in life. Mark and Lexie cope with an arrival of a woman from his past. Thatcher questions the Chief's recent behavior as Meredith comes to his defense.

Mark flies Addison to Seattle to help with a difficult procedure on his pregnant daughter, Sloan, Owen questions Teddy's motives when she assigns Cristina the lead on a complicated surgery, and Derek's suspicions are raised when the Chief recruits Meredith to assist with a high profile operation on.

After learning of Richard's drinking problem, Derek confronts him for the sake of his friend and the hospital, Izzie returns, hoping to reconcile with Alex, and tensions run high between Teddy, Cristina and Owen in the aftermath of Cristina's startling confession.

As Derek begins his role as interim chief, he faces a potential lawsuit when Bailey and Meredith's patient awakens from anesthesia mid-surgery. Meanwhile, Teddy refuses to place Cristina on her service, as Arizona tests Alex out in Pediatrics, and Mark refuses to speak to a heartbroken Lexie.

It's Valentine's Day, and the doctors must treat dozens of injured people after the roof collapses at a popular romantic restaurant; Derek tries to manage his busy schedule as Chief of Surgery; Meredith wrestles with her new duties as the Chief's wife; and Mark and Callie team up to convince Sloan to do the right thing for her baby.

In his new role as Chief, Derek restores the hospital lecture series, and Richard, Bailey and Callie present to the group pivotal surgical cases from their pasts Bailey reflects on her first days as a shy resident, Callie looks back on a polio case, and Richard recalls a case in which he and Ellis worked on a patient diagnosed with GRID, a virus later called AIDS.

When famous surgeon Dr. Harper Avery is rushed to Seattle Grace-Mercy West, the staff is surprised to discover that one of their own is related to this legend, and his unorthodox request for his surgery causes tensions t between Derek and Richard. Meanwhile Callie and Arizona try to help Teddy and Sloan move on from their pasts and find greener pastures.

A patient who needs an extremely difficult surgery comes to Seattle Grace, and it's Richard and Owen who vie for the opportunity to tackle it. But the decision lies in the hands of Derek, who deems that Owen should take on the surgery, leaving Richard bitter. Meanwhile, Bailey deals with her own personal struggles when she nervously prepares for another date with Ben.

When Teddy's patient seeks to end treatment and her life, she turns to Hunt for help. The situation triggers Hunt's memories of his life at war and his past relationship with Teddy. Meanwhile, Callie and Arizona have a difference in opinion on what their future holds, and Richard tries to adjust to his role as a surgeon.

When Alex's younger brother, Aaron, shows up at Seattle Grace-Mercy West with a hernia, it's up to Alex to get Bailey's approval for his pro bono surgery. Matters are further complicated when Aaron discloses that his family has not heard from Alex since he arrived in Seattle, and parts of Alex's past long kept secret are revealed to his friends and fellow doctors.

Sloan returns to Mark's apartment, just as she's going into labor, and Teddy, Mark and the still-fighting Callie and Arizona are there to help with the birth and to get Mark through his grandson's pending adoption. Meanwhile Teddy sees Derek's invitation to illustrious cardiothoracic surgeon Tom Evans' as a threat to her future employment at the hospital, and the team work on a crab boat captain who has been stabbed with a giant shark hook.

Bailey preps the team with mandatory sensitivity training prior to admitting a 700-pound patient with compounded medical issues, and the case proves to be challenging in every sense of the word. Meanwhile Derek has to come face to face with a former patient's husband in a wrongful death deposition, and spending time with a heart patient's daughter opens up some old wounds for Cristina.

An elderly patient admitted into the E.R. for a heart condition sees a familiar face, a long lost love who happens to be in the E.R. as well for a fractured arm, and the staff find themselves caught up in their love story. Meanwhile, Karev treats a troubled teenage patient (guest star Demi Lovato) whose parents brought her in for schizophrenia, and Meredith can't help but tell Cristina about her suspicions of Owen -- which inevitably messes with Cristina's head.

Seattle Grace Hospital is hit with a crisis like no other in its' history.

Cristina and Meredith's surgical skills are put to the ultimate test.