: Seattle Grace residents are forced to compete for surgeries and equipment with their new rivals. Tensions boil over when Reed Adamson attempts to take George O'Malley's old locker, leading to a confrontation with Izzie Stevens.

The fifth episode of the sixth season of , titled " Invasion ," is a pivotal hour centered on the high-tension merger between Seattle Grace and Mercy West hospitals. Originally aired on October 15, 2009, it marks the introduction of significant new characters and a major departure for a long-time series regular. Plot Summary

: Fans often remember this episode for the "dark and twisty" bonding between Cristina Yang and Alex Karev, both of whom seek solace in Meredith's room as they deal with the fallout of the merger.

: Meredith Grey remains largely sidelined in a hospital bed throughout the episode, a creative choice made to accommodate actress Ellen Pompeo’s real-life maternity leave; the character is written as having just donated a portion of her liver to her father. Production and Impact

: Callie Torres faces a personal crisis when her father, Carlos, returns to the hospital with a priest in a misguided attempt to "cure" her bisexuality.

: The episode was written by Mark Wilding and directed by Tony Phelan .

: This episode served as the first half of a crossover event with the spin-off Private Practice . The storyline of the patient Izzie endangered, Sarah Freemont, concludes in the Private Practice episode "Right Here, Right Now".