'Grey's Anatomy' Reveals Eerie Promo for Record-Breaking Episode

Grey's Anatomy is making TV history with this Thursday's new episode.

The show will officially beat ER with Thursday's 332nd episode as the longest medical drama in television history, and Grey's will be celebrating in style.

At the end of Feb. 21 episode, Grey's unveiled the eerie promo to the special hour, featuring snippets of iconic lines from the series and a new voiceover from star Ellen Pompeo.

"You know that old saying, "if these walls could talk," Pompeo says as viewers get a look at the hospital's lobby set and one of the operating rooms. "Join us as Grey's Anatomy becomes the longest-running primetime medical show in all of television."

"Because after 332 episodes, we still have so much more to say," she adds at the end of the cryptic clip.

Despite the seemingly dark tone of the promo, "We Didn't Start the Fire" will actually be an episode unlike many others before it, as the episode will take place entirely outside of the hospital.

An official episode description for the episode released by ABC reads: "When Jackson (Jesse Williams) throws a party to celebrate Catherine's (Debbie Allen) surgery and those who worked to save her, literally nothing goes as planned. Meanwhile, Helen (Lindsay Wagner) pays Alex (Justin Chambers) and Jo (Camilla Luddington) a surprise visit.

The episode will mark the first time Wagner returns since she was introduced as Alex's mother in a Season 14 episode.

Ahead of the record-breaking episode, Grey's Anatomy showrunner Krista Vernoff revealed to The Hollywood Reporter that in honor of the major milestone, the series would do something it has never done before.

"In the 300th episode we did a huge number of winks at the show's history and beginnings. I don't know if ER did it or not but what I came up with was a no-medicine episode," Vernoff said.

Kelly McCreary, who plays Maggie Pierce on the show, told the outlet the special episode feels like a celebration of everything the series has achieved.

"We're in this party scene and I keep waiting for somebody to need a tracheotomy! But instead it's great because it feels like a real celebration of these characters," she said.

Pompeo began the celebration Wednesday with a touching Instagram selfie alongside series creator Shonda Rhimes.

"We started this journey 15 years ago and this weeks episode makes history," the actress wrote. "The truest thing about this journey is we appreciate how incredible it is. We celebrate ALL of it. The ups and downs... the life lessons dance parties and the exhaustion of hard work..making history and being afforded the privilege of being able to. Thanks to everyone who has been on this ride with us... we are still having a ball."

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