Grey's Anatomy postmortem: How will Grey Sloan react to the new resident?

Warning: This story contains major spoilers from Thursday’s episode of Grey’s Anatomy. Read at your own risk!

Come Monday, someone’s going to have a really tough first day of work.

During Thursday’s special 250th episode of Grey’s Anatomy, Meredith (Ellen Pompeo) hosts what will now be called the dinner party from hell. Shocked to discover that Callie’s (Sara Ramirez) new girlfriend is Penny (Samantha Sloyan), the woman partially responsible for Derek’s (Patrick Dempsey) death, Mer accepts that if she can make it through this dinner, she’ll never have to see Penny again. That is, until she learns that Penny is transferring to Grey Sloan as a new resident. While Mer is… let’s say, less than pleased, Amelia (Caterina Scorsone) is even more devastated — something that will definitely set the sisters’ relationship back. How will the rest of Grey Sloan react? And does this spell the end for Callie and Penny? EW caught up with executive producer Debbie Allen to get the scoop:

ENTERTAINMENT WEEKLY: How will Penny’s arrival at Grey Sloan shake things up?

DEBBIE ALLEN: There’s already been a wrecking ball, and it remains to be seen how this is going to work out, because everybody loved Derek. Everybody knows how devastated Meredith has been after leaving for a whole year. Everybody embraced her, and now they’re going to look at [Penny] a little differently. Some people will be sympathetic, some people just can’t stand to look at her. It’s going to be tough.

Will Penny find any allies at Grey Sloan?

I would say anybody can find allies, it’s just going to take a matter of time. How long she’s going to be here, I have no idea, because we never know what Shonda Rhimes is going to do. We’re in episode 9, and it’s its own animal right now. We can’t be sure where anything is going to go.

How is Meredith dealing with Penny being around at the hospital?

It’s a constant reminder. It’s very difficult. How can she see her without thinking about Derek? This is a journey for Meredith of personal growth. Is she going to rise, or stay in that dark place? That’s what we’re going to play out.

Will Penny ever be able to actually explain that she tried to do the right thing with Derek?

She explained it to Meredith when she was at the hospital. I don’t think Penny wants to talk about this at all. How can you explain it? He’s dead. Whatever you say, it’s not going to bring him back. There’s Meredith, a widow with three children. How do you fix that? You can’t. Penny is coming to Grey Sloan because she wants to be a better doctor. This is a teaching hospital, which is what’s so wonderful about this season — all the wonderful interns and residents. We’re going back to the early Grey’s in terms of the energy and the lightness in the midst of very heavy stories and medical crises and challenges.

Is Callie torn between Penny as her new girlfriend and Meredith as one of her best friends?

Yeah, that’s really tough. It’s great drama. The audience is going to be riveted. I think our viewership is going to triple. [Laughs] It’s so amazing what’s going on. Our stories are so based on such real life situations that are not improbable at all. They’re very universal stories. What happens to a family when a wrecking ball comes in? How does everybody react? Who’s sympathetic? Who’s not? Not everyone has the same reaction.

Will people be upset with Callie because of who she’s dating?

Amelia’s already very upset. Amelia is out of her mind about it. That’s evident. That was her brother, it’s very real for her. She had so many personal life crises. She’s recovered from addiction — Derek was always the one to help her through all that, and now this happens. She was in a good mood at the party, she was happy.

With Amelia so upset and Meredith just trying to put it aside, what does that do to their relationship?

It’s a wrecking ball. You heard what Meredith said to Owen (Kevin McKidd): Get her out of this room before I kill her. She was so upset. It was raw. The gloves have come off. There’s no sugarcoating any goddamn thing. That’s amazing drama. Even sisters who have grown up together and love each other could fall out over something like this. It can do a lot of things.

Grey’s Anatomy airs Thursdays at 8 p.m. ET on ABC.

Watch the casts of Grey’s, Scandal, and How to Get Away with Murder reveal which Shondaland deaths devastated them the most: