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 Callie Torres (Sara Ramirez)'s father returning to the hospital to condemn her bisexuality, and Izzie Stevens (Katherine Heigl) putting her career into jeopardy after administering the wrong treatment to a patient.

Héctor Elizondo returned as a guest star, while Sarah Drew, Jesse Williams, Nora Zehetner and Robert Baker made their first appearances.

"Invasion" received positive reviews, with critics praising Drew's, Heigl'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 The patient Sarah Freemont (Alexie Gilmore)'s life hangs in the balance after Izzie's dialysis error.

Her arc concludes in the second-half, a Private Practice episode titled "Right Here, Right Now", which sees Miranda Bailey (Chandra Wilson) fly Freemont to Addison Montgomery in LA for help.

The episode opens with a voice-over narration from Meredith Grey (Ellen Pompeo) about confronting challenges and deciding whether to fight, adapt, or surrender when faced with overwhelming obstacles.

Despite her mindset that all Mercy West residents would act in the manner of Adamson, Stevens develops a friendship with Charles Percy (Robert Baker).

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 named Sarah Freemont (Alexie Gilmore), 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.

Having found out about Stevens' mistake, Miranda Bailey (Chandra Wilson) reprimands her and reports her to the chief of surgery Richard Webber (James Pickens Jr.).

[8] Michael Pascua of The Huffington Post praised the scene in which Yang broke down to Meredith, noting the exceptional chemistry between them.