In the show, some filmmakers will arrive at Seattle Grace to make a programme about the aftermath of the shootings. The episode is all from the point of view of the cameramen.
"You actually feel like you're getting a behind-the-scenes look at our characters' lives," Rhimes told Entertainment Weekly. "You see our characters in a very different way because we're approaching them in a very different way."
However, Rhimes added that the episode will also advance the storylines.
She revealed that Alex (Justin Chambers) works out what kind of surgeon he wants to be, adding: "It seems [paediatrics] is his thing."
Elsewhere, Callie (Sara Ramirez) and Arizona (Jessica Capshaw) will make "a huge decision" that moves them "into a new chapter". Meanwhile, viewers will learn more about Cristina (Sandra Oh) and her struggle with post-traumatic stress disorder.
"She has a long journey," Rhimes explained. "But when she gets a little bit better, she comes roaring back."
Rhimes also revealed that she does not want the episode to provide "closure", saying: "Closure is the wrong word for every last one of my characters. I spent a lot of time really trying to get this right, speaking to grief counsellors and reading the Columbine book and really thinking about this, and not wanting to minimise this. I don't think there's such a thing as closure. I think you move on and become different, and you're changed, but I don't think it's ever closed."
Grey's Anatomy continues on Thursdays at 9/8c on ABC.