The actress, who plays April Kepner in the ABC series, said that a big event in the May 17 episode will leave the characters with "an uncertain future".
"It's on par with the shooter finale," she told TV Guide. "It will affect everyone, but it happens outside of the hospital.
"The people that are at the hospital don't know it's happened yet by the end of the episode - I'm one of the people who is back at the hospital. Everybody is left with an uncertain future."
It has long been rumoured that cast members Patrick Dempsey, Ellen Pompeo and Sandra Oh will depart the show after season eight.
Grey's Anatomy airs Thursdays at 9/8c on ABC.
> 'Grey's Anatomy' exec Shonda Rhimes hints at cast exits