<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> <P> The episode was written by Mark Wilding and directed by Tony Phelan . Jenny Barak edited the episode's music and Donald Lee Harris served as production designer . Eric Dane (Dr. Mark Sloan) was absent from the episode . Featured music included The Myrmidon's "Clap", Traildriver's "I Want You Now", and Greg Laswell's remake of "Your Ghost". "Invasion" was the last episode Heigl appeared in, before her five - episode hiatus to film the romantic comedy Life As We Know It . </P> <P> Wilding called the guest stars "a blast to work with". He deemed the theme of the episode "invasion", adding that all the character's lives were being invaded . In the episode, Stevens makes a mistake, following a disagreement with Percy . Wilding commented that Stevens was originally seeking friendship with Percy, because he reminded her of O'Malley . At the conclusion of the episode, Torres ultimately reunites with her father Carlos . Wilding offered his insight on this and the overall episode: </P> <P> "Shonda came up with the idea of Mr. Torres showing up with the family priest . I immediately said YES because anytime I can see Hector Elizondo on my TV screen, I'm all for it . In the scene where he's quoting the Bible at Callie, he really is afraid he's going to lose her . He's desperate, yanking out those index cards like her very life depends on it . It was Hector's idea to use the cards . He didn't think his character would know that stuff of the top of his head, which is how I originally wrote it . And he was right . And it worked beautifully . And that final scene when he and Callie are outside the hospital and he's asking her if she'll still get married, still have kids...I really, really liked that scene . And I really, really liked their whole story . All told, I thought the episode came out pretty well ." </P>

When do the two hospitals merge in grey's anatomy