Grey's Anatomy fans could never forget about MerDer, but Ellen Pompeo's maternity leave the first half of this season has relegated the show's core couple to the back burner.

What's up next for Mer and Der when Grey's Anatomy returns in just over a week? Not good things. Don't be alarmed, they're together for good. But D stands for doghouse.

That's because he learns Rule #1 of marriage the hard way when he lets slip something Mer told him confidence. At work. To people Mer also knows. Women don't like that.

But here's where it gets intriguing: The secret's impact "will be much more wide-ranging at Seattle Grace-Mercy West" than just a case of marital annoyance, reports say.

What do you think the big secret is?

Meredith won't be smiling at her husband for long, it appears.

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.