Get ready for another heartbreaking hour of Grey's Anatomy in the Season 8 finale when the docs must rally on what could be their last day working together — as many of Seattle Grace's familiar faces will have already accepted jobs at other hospitals!

"There is a very complex thing that occurs for many of our characters that places a lot of them in a position to discover what they're made of," the ever-tight-lipped Grey's boss Shonda Rhimes teases of the major event that erupts in the season-ender. (Seriously, she said "thing.") "I don't know that you could guess it," she says.

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Challenge accepted! Could it be a forest fire that threatens to claim Meredith (Ellen Pompeo) and Derek's (Patrick Dempsey) recently finished McMansion? Maybe a rescue helicopter will fall from the roof and crush one of the docs? (Wait a minute; that sounds familiar.) Rhimes' only clue: When we asked whether this incident is similar to the devastating ferry boat crash or the rampaging shooter from seasons past, she replied, "Yes. That's a yes to both!"

In a year where contract negotiations of key cast members are also still up in the air, Rhimes has appeared to create a perfect out should some of our favorite doctors not return. "People will take jobs at different hospitals," Rhimes says, noting that the docs will make their decisions in the penultimate episode. We've already seen that Cristina (Sandra Oh) is the most eager to get out of Dodge, but, in a surprising twist, we learned last week that Meredith's long-time home is not her first choice. Who may leave? "The plan is, at the end of the season, our doctors will do what residents all over the country do, which is to accept jobs at different hospitals," she says. Could some docs stick around Seattle Grace? "Not at the end of the season," Rhimes teases, noting that one of many possibilities is that some doctors will return in the potential ninth season, should ABC decide to renew the series.

With that in mind, we wondered: Is someone going to die? The medical drama has never shied away from killing off main characters — it takes place in a hospital, after all — but Rhimes was coy about whether or not the event would bring the Grim Reaper to town. "I don't want to answer this question," she said bluntly, only noting that the event would be dangerous. "It's very dark and twisty," she said. "It was really hard [to write] because I'm all happy; I have a baby [now]."

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"We're taking our characters on a journey they've never been on before," she adds. "To me, it's a very shocking thing that I chose to do and I was really stressed-out about doing it, and it took me a long time, so I kept taking the finale apart and putting it back together. It's also a very emotional episode. I always feel like when an episode really scares me, when I spend a lot of time with my head in my hands wondering how the fans are going to take it, and worry that they're going to, I don't know, stone me in the streets, I kind of know that it's good."

In that sense, this finale will be almost a polar opposite to last year's, which saw Meredith and Derek hit another rough patch when she tampered with the Alzheimer's clinical trial. "I feel like last year's finale was very quiet for a reason," Rhimes says. "Our characters had been through so much in terms of having dealt with the shooting and getting over that and coming to a place where really their decisions were about their growth as people — Meredith and her baby and Cristina and her pregnancy. This season, I feel like the theme of the episode is graduation, and there is a huge sense of nostalgia in this episode in terms of what you're looking at. This could very well be the last time these doctors are working together. This could very well be the last day for some of these doctors here."

Still, she adds, "There are some really beautiful moments. There's a great moment for Bailey [Chandra Wilson] that I think is light, wonderful, beautiful, romantic and amazing that I'm very excited about. But there are some very, very dark things happening."

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Some of that darkness will surely come from the tumultuous marriage between Owen (Kevin McKidd) and Cristina, which has seen many gut-wrenching moments already. "I really do believe and love them together as a couple," she says. "When I say, 'Oh, I want them to stay together,' people think that means that they're going to hold hands and skip happily off into the sunset. We all know that's never going to happen or else we wouldn't have a television show. I do ultimately think that they belong together, but that road could be very long, or it could be short.

"I haven't gotten into next season yet," she continues. "I can't even think past how we're ending this season. And when we end the season, so many other things are happening that I'm not sure that you're going to spend time worrying about whether or not Cristina and Owen are going to be together. In the episode before the finale, you will feel like you know what's going to happen to them, but their journey really does continue."

With Rhimes confident that Grey's will return for a ninth season, will there still be a sense of finality should some of the doctors bid farewell? "There are some things that will feel like there's some closure and many things that won't."

Chew on that.

The Grey's Anatomy finale airs Thursday, May 17 at 9/8c on ABC.