On last week's episode of "Grey's Anatomy," "Old Time Rock and Roll ," guests seemed to be enjoying themselves at Meredith, Maggie and Amelia's dinner party, until Callie showed up with her new girlfriend, Penny. Although it was assumed everyone -- minus Arizona -- would be thrilled for Callie, Meredith couldn't help but associate her friend's new love interest with Derek's death.

As we'll learn this week, it turns out Penny was one of the doctors who couldn't save Derek's life.

In the sneak peek video above, Meredith deals with her reaction to Penny in the midst of hosting a party with her colleagues.

Grey's Anatomy" Season 12, Episode 5, "Guess Whos Coming to Dinner?, airs Oct. 22 at 8 p.m. on ABC.

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