Actors Ellen Pompeo, Sandra Oh, Katherine Heigl, and Justin Chambers reprised their roles as surgical residents Meredith Grey, Cristina Yang, Izzie Stevens, and Alex Karev, respectively.

Main cast members Patrick Dempsey, Chandra Wilson, James Pickens, Jr., Sara Ramirez, Eric Dane, Chyler Leigh, and Kevin McKidd also returned, while previous recurring-star Jessica Capshaw was promoted to a series-regular, and Kim Raver was given star-billing after the commencement of the season.

Further storylines include Shepherd being promoted to Chief of Surgery and Seattle Grace Hospital merging with the neighboring Mercy West, which introduces several new doctors.

The season culminates in a life-threatening situation when a grieving deceased patient's husband embarks on a shooting spree at the hospital, seeking revenge for his wife's death.

Cristina, Izzie, Lexie, and Alex, are introduced to and clash with their Mercy West counterparts and competition: Jackson Avery (Jesse Williams), Charles Percy (Robert Baker), April Kepner (Sarah Drew), and Reed Adamson (Nora Zehetner).

Meanwhile, Arizona stands by an enraged Callie when her father visits her with a priest to convince her to renounce her homosexuality; also, an accident derails a patient's upcoming kidney donation, with Bailey promising to set things right.

Absent: Eric Dane as Mark Sloan Following the death of a patient, all the doctors concerned with the case are interviewed in a who-dunnit style.

Owen, Cristina, Bailey, Alex, Callie, Lexie, Charles, Jackson, and Reed all give their opinions, and as the story unfolds to show that April Kepner was the one whose mistake it was, she is fired, even though Derek is against it.

Cristina tests her relationship when she blatantly disregards Owen's orders and performs an unauthorized cardiac procedure, but after encouragement from Meredith and a kiss from Jackson, she decides to try and save it.

The holidays aren't so merry for the doctors at Seattle Grace-Mercy West hospital when Cristina suspects that something is or has been going on between her boyfriend, Owen, and his old friend, Teddy.

Bailey is stunned when her father makes rash judgments about her life and the choices she has made, finally culminating in a showdown at Meredith's Christmas dinner.

Absent: Katherine Heigl as Izzie Stevens As Derek begins his role as interim chief, he faces a potential lawsuit when Bailey and Meredith's patient awakens from anesthesia mid-surgery.

Arizona tries to help Teddy move on to greener pastures when she sees how obsessed she is with Owen, while she struggles to work with Alex after learning that he'd once slept with Callie.

Mark attempts to move on after giving up Sloan's baby through sex, and tells Derek and Callie he can restore a patient's hearing.

The season was produced by Touchstone Television ABC Studios, The Mark Gordon Company, Shondaland and was distributed by Buena Vista International, Inc.

The regular directors were Shonda Rhimes, Krista Vernoff, Stacy McKee, William Harper, Debora Cahn, Allan Heinberg and Peter Nowalk.

[28] Heigl explained that she wanted to spend more time with her family, and did not think it would be respectful to Grey's Anatomy viewers to have Izzie return and depart yet again.

"[30] The episode's writer, Heinberg, offered his insight: "Teddy's arrival at Seattle Grace unleashes all manner of complications for Owen, Cristina, and herself.

Series' writer Joan Rater commented on this: "Izzie getting the clean scan back gives Alex the freedom to leave.

Fellow third-year residents Cristina Yang, Izzie Stevens and Alex Karev are portrayed by Sandra Oh, Katherine Heigl and Justin Chambers, respectively.

Attending general surgeon Miranda Bailey was portrayed by Chandra Wilson whose main storylines throughout the season focus on her divorce and the development of new romantic relationships.

Seattle Grace Hospital's Chief of Surgery and general surgeon Richard Webber was portrayed by James Pickens, Jr., who returns to alcoholism after being sober for 20 years.

Mercy West surgical residents Reed Adamson, Charles Percy, April Kepner and Jackson Avery were portrayed by Nora Zehetner, Robert Baker, Sarah Drew and Jesse Williams, respectively.

Thatcher Grey (Jeff Perry) and Sloan Riley (Leven Rambin) have been part of the season's main story arcs, while numerous episodic characters have made guest appearances: Demi Lovato as Hayley, Sara Gilbert as Kim Allen, Marion Ross as Betty, Mandy Moore as Mary Portman, Ryan Devlin as Bill Portman, Nick Purcell as Doug, Michael O'Neill as Gary Clark, Danielle Panabaker as Kelsey, Adrienne Barbeau as Jodie Crawley, Héctor Elizondo as Mr. Torres, Cody Christian as Brad Walker, Amy Madigan as Dr. Wyatt, and Missi Pyle as Jasmine.

Former series-regular Kate Walsh returned to the series as a special guest-star, portraying neonatal surgeon and obstetrician-gynecologist Addison Montgomery.

Speaking of the premiere, Glenn Diaz of BuddyTV noted that the special foreshadowed a "very dark" season, adding: "The talk between George's mom and one of the surgeons [Torres] proved to be one of the more heart-breaking scenes in an episode that in itself is heartbreaking enough.

That said, this is Grey's Anatomy and with that comes the usual drama, sex, love and whacky medical mysteries thrown in the mix to keep things moving.

[41] TV Fanatic's reaction to the season was fairly mixed,[42][43] with Steve Marsi saying that Grey's Anatomy was facing an identity crisis after viewing "Give Peace a Chance".

"[44] He praised Patrick Dempsey's performance, saying: "Again, it was a single case that took up the entire hour, but instead of 12 doctors' version of events, the focus was largely on just one, and the best one: Dr. Derek Shepherd.

Seriously, the Season 6 finale left us laying awake afterward thinking about everything, a feeling we haven't had from Grey's in years and rarely achieved by any program.