"Invasion" is the fifth episode of the sixth season of the American television medical drama Grey's Anatomy, the show's 107th episode overall, and the first of a two-part patient crossover event with Private Practice. 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 's father returning to the hospital to condemn her bisexuality, and Dr. Izzie Stevens putting her career into jeopardy after administering the wrong treatment to a patient.

The sixth season of the American television medical drama Grey's Anatomy, commenced airing on the American Broadcasting Company (ABC) in the United States on September 24, 2009, and concluded on May 20, 2010. The season was produced by ABC Studios, in association with Shondaland Production Company and The Mark Gordon Company; the showrunner being Shonda Rhimes and head writer Krista Vernoff. 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. Heigl was released from her contract in the middle of the season, while T. R. Knight did not appear as George O'Malley, because Knight was released from his contract at the conclusion of season five. 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.