<P> "Invasion" is the fifth episode of the 6th season of the American television medical drama Grey's Anatomy, and the show's 107th episode overall . It was written by Mark Wilding and directed by Tony Phelan . The episode was originally broadcast on the American Broadcasting Company (ABC) in the United States on October 15, 2009 . In the episode, the physicians of Seattle Grace Hospital must learn to co-operate with new Mercy West residents, who do not act in a kind manner to them . Other storylines include Dr. Callie Torres (Sara Ramirez)'s father returning to the hospital to condemn her homosexuality, and Dr. Izzie Stevens (Katherine Heigl) putting her career into jeopardy after administering the wrong treatment to a patient . </P> <P> The episode was Heigl's last appearance before her five - episode hiatus to film a movie . It is set in a fictional hospital in Seattle, Washington . Héctor Elizondo returned as a guest star, while Jesse Williams, Sarah Drew, Robert Baker, and Nora Zehetner made their first appearances . "Invasion" received generally positive reviews, with critics praising Drew's, Jessica Capshaw's, Ellen Pompeo's, and Sandra Oh's performances . Upon its initial airing, the episode in the United States was viewed by 13.79 million people, garnered a 5.0 / 13 Nielsen rating / share in the 18--49 demographic and ranked third for the night in terms of viewership . </P> <P> The hospital receives several surgical residents from Mercy West as they enter Seattle Grace, a repercussion of the recent merger . Dr. Meredith Grey (Ellen Pompeo) is recovering from her liver transplantation surgery, after donating a portion of it to her father . In the residents' lounge, Dr. Izzie Stevens (Katherine Heigl) tries to explain to Mercy West resident Dr. Reed Adamson (Nora Zehetner) that she should not use a certain locker, due to it formerly belonging to the deceased Dr. George O'Malley (T.R. Knight). Adamson ignores her wishes, and uses the locker, leading Stevens to threatening to fight her . Shortly thereafter, Dr. Cristina Yang (Sandra Oh) and Dr. Lexie Grey (Chyler Leigh) are working in the emergency room, when Dr. Owen Hunt (Kevin McKidd) embarrasses Yang by taking Mercy West resident Dr. Jackson Avery (Jesse Williams)' side in a disagreement . Despite her mindset that all Mercy West residents would act in the manner of Adamson, Stevens develops a friendship with Dr. Charles Percy (Robert Baker). After months of the two not speaking, Dr. Callie Torres (Sara Ramirez)'s father, Carlos (Héctor Elizondo), shows up to the hospital with a priest, trying to condemn Torres for her concurrence in homosexuality . </P> <P> Under the supervision of Dr. Derek Shepherd (Patrick Dempsey), Lexie works with Mercy West resident Dr. April Kepner (Sarah Drew) on a burglar, and the two get in several disagreements, leading Lexie to steal her diary . Dr. Alex Karev (Justin Chambers) and Adamson continuously argue over the course of treatment for a patient, leading the patient's daughter to yell at them in disbelief . Angry about the revelation that Percy (who later apologized saying that they can be friends outside the hospital, but not inside) was using her, Stevens accidentally administers the wrong dialysis to a patient, making her an unviable candidate for the kidney transplant she was scheduled to receive . After arguing over surgical cases for the entirety of the day, Yang comes to the realization that she needs a cardiothoracic surgeon to work at the hospital . After being mistreated by Kepner, Lexie begins to mock her about what is written in her diary, thus ruining her emotionally, but subsequently apologizes . Dr. Arizona Robbins (Jessica Capshaw), Torres' girlfriend, talks to Carlos, and convinces him to accept his daughter's sexuality . Having found out about Stevens' mistake, Dr. Miranda Bailey (Chandra Wilson) reprimands her reports her to the chief of surgery Dr. Richard Webber (James Pickens, Jr .). Webber asks Stevens to come into her office, and he fires her from the staff, noting that Hunt, Shepherd, and Karev had concerns with her coming back to work so early . Stevens departs, writing Karev a Dear John letter, causing him to cry in front of Meredith and Yang . </P>

Name the hospital that seattle grace merged with in season 6