Did Derek cheat on Meredith? That is the question on everyone's mind, including Derek's wife who is already convinced infidelity has occurred in their volatile relationship.

Brace yourself, because Derek will finally address his alleged affair in "Grey's Anatomy" Episode 17, "With or Without You." He will explain why a mysterious woman answered his phone the night Meredith called. No matter what excuse the handsome doctor gives, it doesn't seem likely Meredith will accept it.

In the video clip, viewers get a glimpse of how Meredith intends to handle Derek. "There's this thing I used to do when I was a kid to my mom's VCR," Meredith says in the promo video. "I'd take it apart piece by piece then put it back together again." It seems Meredith has every intention of carefully dissecting Derek's story for inconsistencies on Thursday night.

"I left my phone at the lab. By the time I figured out I left it, I was already at the airport coming to you," Derek says. But that doesn't explain why a random woman would answer his phone, does it? Meredith asks that very question.

"I don't know," Derek responds. "She probably thought it was me. I called her from the airport, then she told me she spoke to you."

Viewers may remember in Episode 15 that Meredith called her husband to cheerfully tell him that she wanted to share her success with him rather than alone, but the happy occasion was interrupted when a strange woman picked up his cell phone. "Who is this?" Meredith demanded again and again. The woman refused to give her name. Instead, she quickly hung up on Meredith.

According to Derek, the mysterious woman is simply his research fellow.

IMDb reports that actress Scottie Thompson will play Derek's Washington, D.C.-based fellow, Renee Collier, who answered the phone when Meredith called.

"I wish I could believe you," Meredith says. "I know how this works. I was your resident once. Seems like you came home because you thought you were in trouble."

"I came home because we're in trouble," he says, referring to their marriage.

"Grey's Anatomy" Episode 17 "With or Without You" airs on Thursday, March 26, at 8 p.m. EDT on ABC.

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