Grey's Anatomy Grey's Anatomy

A shooter may not be rampaging through Seattle Grace, but that doesn't mean there won't be casualties in the season finale of Grey's Anatomy. This time, however, it's the doctors' relationships that end up on life support — and may even flatline.

"Last year, the finale was about something external coming in and forcing our characters to cling to one another for dear life," Sarah Drew tells TVGuide.com. "This finale is about something internally that implodes and ends up breaking relationships and tearing people apart."

Shonda Rhimes on the Grey's Anatomy finale: No gunmen, more growth

Who's to blame? Well, Meredith (Ellen Pompeo) first set the events of the finale into motion in the episode "It's a Long Way Back," when she tampered with the Alzheimer's clinical trial packets, ensuring the Chief's wife, Adele (Loretta Devine), would get the actual medicine and not the placebo. Though she believed it was the right thing to do at the time, Meredith jeopardized not only the trial, but the hospital and her marriage, as well as her impending adoption of Zola.

Afraid that Meredith would edge him out in the race for Chief Resident, a drunken Alex (Justin Chambers) revealed her misdeed to Owen (Kevin McKidd). "Alex is going to have to deal with the fallout of making the choice to rat her out," Drew says. "He walks into the decision hoping to accomplish one thing and ends up accomplishing something completely different, which creates a huge storm of drama between him and Meredith.

"Cristina [Sandra Oh] also finds out, so there is also drama between Cristina and Meredith," Drew adds, noting that most of the hospital will be unaware of Meredith's indiscretion. Unfortunately, Derek (Patrick Dempsey), who only officially married Meredith recently, does find out and isn't pleased at all. "All these relationships that we thought were so strong, we're going to see them all crack and crumble."

Grey's Anatomy relationship roundup: What's next for the couples of Seattle Grace?

Cristina has more to contend with than a friend who betrayed her trust though. "You discover she's pregnant," Drew says. "It's going to create a huge amount of drama between Cristina and Owen. It became clear a few episodes ago that he was really excited to have children and she was like, 'Are you kidding? Do you know me? No, never.' That is going to present a huge issue in their relationship and marriage."

Amidst all this drama, the race for Chief Resident will come to a head. We've already seen a few doctors pull out — Cristina because she wants to focus on surgery and Jackson because he dropped out of the Chief's trial. "We just have Meredith and Alex left over," Drew says, noting that we shouldn't count April out of the race yet. "Alex has just tattled on Meredith and she has just screwed with the Alzheimer's trial, but there will still be tons and tons of drama associated with this whole run for Chief Resident."

The season finale of Grey's Anatomy airs Thursday at 9/8c on ABC.