The episode, titled 'If/Then', explores an alternate universe of Seattle Grace as Meredith (Ellen Pompeo) thinks about what might have happened had her mother remained alive. It will feature the return of Private Practice star Kate Walsh.
This season of Grey's Anatomy could be the last for both Pompeo and Patrick Dempsey, whose contracts with the show expire this year.
Both stars, who have been key players on the medical drama since 2005, say they plan to pursue careers in film after their departure.
'If/Then' airs this Thursday (February 2) at 9/8c on ABC.
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Watch the preview of the new episode below: