<Tr> <Td> ← Previous "Tainted Obligation" </Td> <Td> Next → "I Saw What I Saw" </Td> </Tr> <Tr> <Td_colspan="2"> Grey's Anatomy (season 6) List of Grey's Anatomy episodes </Td> </Tr> <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>

When did mercy west merge with seattle grace