Did Derek (Patrick Dempsey) cheat on Meredith (Ellen Pompeo)? That seems to be the question on everyone’s mind – including Derek’s wife, who is convinced there’s been foul play in their already tumultuous relationship.

Derek will finally come clean in episode 17 about his alleged affair and explain why a mysterious woman with a perky voice answered his phone the night Meredith called. Curious what excuse Dr. McDreamy will use? Well, whatever explanation Derek has up his scrub sleeve, it doesn’t seem like Meredith will buy it.

In a sneak peek clip from “With Or Without You,” viewers can see Mer’s suspicions of her husband’s tall tale. “There’s this thing I used to do when I was a kid to my mom’s VCR,” Meredith narrates the promo video. “I’d take it apart piece by piece then put it back together again.” It looks like Meredith will be meticulously dissecting Derek’s story for holes in the next installment of the hit ABC series.

“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 in an attempt to explain why his phone was left in the hands of a stranger. But Meredith will be more interested in why this random woman felt the need to answer her husband’s phone.

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

In episode 15 Meredith called her husband to tell him that she’d rather share her success with him than bask it in alone. But her cheerful mission was overshadowed the moment another woman picked up her husband's cell. “Who is this?” Meredith demanded over and over. But the woman on the other end refused to give her name. Instead, she quickly – and suspiciously – hung up on Meredith.

“Is that it? Is that everything you have to say?” Meredith questions, doubting her husband’s recount. Derek, however, claimed there was nothing more to his story.

Meredith reveals what Derek’s connection to the mysterious woman is. “And she’s your research fellow. Just your research fellow,” she confirms. According to reports, actress Scottie Thompson will portray Derek’s D.C.-based fellow, Renee Collier – the character who answered the phone when Meredith called.

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

Derek reveals that he didn’t take the earliest flight home out of guilt but because he wanted to save his marriage. “I came home because we're in trouble,” he says.

Do you think Derek betrayed Meredith? Sound off in the comments section below with your theories before episode 17 of “Grey’s Anatomy” airs on Thursday, March 26, at 8 p.m. EDT.